diff --git a/src/registrar/fixtures/fixtures_user_portfolio_permissions.py b/src/registrar/fixtures/fixtures_user_portfolio_permissions.py
index 0ff740573..22d6a0c0e 100644
--- a/src/registrar/fixtures/fixtures_user_portfolio_permissions.py
+++ b/src/registrar/fixtures/fixtures_user_portfolio_permissions.py
@@ -1,4 +1,5 @@
 import logging
+import random
 from faker import Faker
 from django.db import transaction
 
@@ -51,23 +52,24 @@ class UserPortfolioPermissionFixture:
 
             user_portfolio_permissions_to_create = []
             for user in users:
-                for portfolio in portfolios:
-                    try:
-                        if not UserPortfolioPermission.objects.filter(user=user, portfolio=portfolio).exists():
-                            user_portfolio_permission = UserPortfolioPermission(
-                                user=user,
-                                portfolio=portfolio,
-                                roles=[UserPortfolioRoleChoices.ORGANIZATION_ADMIN],
-                                additional_permissions=[UserPortfolioPermissionChoices.EDIT_MEMBERS],
-                            )
-                            user_portfolio_permissions_to_create.append(user_portfolio_permission)
-                        else:
-                            logger.info(
-                                f"Permission exists for user '{user.username}' "
-                                f"on portfolio '{portfolio.organization_name}'."
-                            )
-                    except Exception as e:
-                        logger.warning(e)
+                # Assign a random portfolio to a user
+                portfolio = random.choice(portfolios)  # nosec
+                try:
+                    if not UserPortfolioPermission.objects.filter(user=user, portfolio=portfolio).exists():
+                        user_portfolio_permission = UserPortfolioPermission(
+                            user=user,
+                            portfolio=portfolio,
+                            roles=[UserPortfolioRoleChoices.ORGANIZATION_ADMIN],
+                            additional_permissions=[UserPortfolioPermissionChoices.EDIT_MEMBERS],
+                        )
+                        user_portfolio_permissions_to_create.append(user_portfolio_permission)
+                    else:
+                        logger.info(
+                            f"Permission exists for user '{user.username}' "
+                            f"on portfolio '{portfolio.organization_name}'."
+                        )
+                except Exception as e:
+                    logger.warning(e)
 
             # Bulk create permissions
             cls._bulk_create_permissions(user_portfolio_permissions_to_create)
diff --git a/src/registrar/templates/domain_add_user.html b/src/registrar/templates/domain_add_user.html
index b2f9fef24..fa3f8e821 100644
--- a/src/registrar/templates/domain_add_user.html
+++ b/src/registrar/templates/domain_add_user.html
@@ -4,11 +4,31 @@
 {% block title %}Add a domain manager | {% endblock %}
 
 {% block domain_content %}
+  {% block breadcrumb %}
+    {% url 'domain-users' pk=domain.id as url %}
+    <nav class="usa-breadcrumb padding-top-0" aria-label="Domain manager breadcrumb">
+      <ol class="usa-breadcrumb__list">
+        <li class="usa-breadcrumb__list-item">
+          <a href="{{ url }}" class="usa-breadcrumb__link"><span>Domain managers</span></a>
+        </li>
+        <li class="usa-breadcrumb__list-item usa-current" aria-current="page">
+          <span>Add a domain manager</span>
+        </li>
+      </ol>
+    </nav>
+  {% endblock breadcrumb %}
   <h1>Add a domain manager</h1>
-
-  <p>You can add another user to help manage your domain. They will need to sign
-  in to the .gov registrar with their Login.gov account.
+{% if has_organization_feature_flag %}
+  <p>
+    You can add another user to help manage your domain. Users can only be a member of one .gov organization, 
+    and they'll need to sign in with their Login.gov account.
   </p>
+{% else %}
+ <p>
+    You can add another user to help manage your domain. They will need to sign in to the .gov registrar with 
+    their Login.gov account.
+ </p>
+{% endif %}
 
   <form class="usa-form usa-form--large" method="post" novalidate>
     {% csrf_token %}
diff --git a/src/registrar/templates/domain_users.html b/src/registrar/templates/domain_users.html
index 412f4ee73..c41902c6f 100644
--- a/src/registrar/templates/domain_users.html
+++ b/src/registrar/templates/domain_users.html
@@ -8,8 +8,7 @@
 
   <p>
   Domain managers can update all information related to a domain within the
-  .gov registrar, including contact details, senior official, security
-  email, and DNS name servers.
+  .gov registrar, including including security email and DNS name servers.
   </p>
 
   <ul class="usa-list">
@@ -17,7 +16,8 @@
     <li>After adding a domain manager, an email invitation will be sent to that user with
       instructions on how to set up an account.</li>
     <li>All domain managers must keep their contact information updated and be responsive if contacted by the .gov team.</li>
-    <li>Domains must have at least one domain manager. You can’t remove yourself as a domain manager if you’re the only one assigned to this domain. Add another domain manager before you remove yourself from this domain.</li>
+    <li>All domain managers will be notified when updates are made to this domain.</li>
+    <li>Domains must have at least one domain manager. You can’t remove yourself as a domain manager if you’re the only one assigned to this domain.</li>
   </ul>
 
   {% if domain.permissions %}
diff --git a/src/registrar/utility/errors.py b/src/registrar/utility/errors.py
index cfa6a537b..e70f06d1e 100644
--- a/src/registrar/utility/errors.py
+++ b/src/registrar/utility/errors.py
@@ -23,6 +23,15 @@ class InvalidDomainError(ValueError):
     pass
 
 
+class OutsideOrgMemberError(ValueError):
+    """
+    Error raised when an org member tries adding a user from a different .gov org.
+    To be deleted when users can be members of multiple orgs.
+    """
+
+    pass
+
+
 class ActionNotAllowed(Exception):
     """User accessed an action that is not
     allowed by the current state"""
diff --git a/src/registrar/views/domain.py b/src/registrar/views/domain.py
index db0572bb3..de156598a 100644
--- a/src/registrar/views/domain.py
+++ b/src/registrar/views/domain.py
@@ -21,8 +21,10 @@ from registrar.models import (
     DomainRequest,
     DomainInformation,
     DomainInvitation,
+    PortfolioInvitation,
     User,
     UserDomainRole,
+    UserPortfolioPermission,
     PublicContact,
 )
 from registrar.utility.enums import DefaultEmail
@@ -35,9 +37,11 @@ from registrar.utility.errors import (
     DsDataErrorCodes,
     SecurityEmailError,
     SecurityEmailErrorCodes,
+    OutsideOrgMemberError,
 )
 from registrar.models.utility.contact_error import ContactError
 from registrar.views.utility.permission_views import UserDomainRolePermissionDeleteView
+from registrar.utility.waffle import flag_is_active_for_user
 
 from ..forms import (
     SeniorOfficialContactForm,
@@ -778,7 +782,18 @@ class DomainAddUserView(DomainFormBaseView):
         """Get an absolute URL for this domain."""
         return self.request.build_absolute_uri(reverse("domain", kwargs={"pk": self.object.id}))
 
-    def _send_domain_invitation_email(self, email: str, requestor: User, add_success=True):
+    def _is_member_of_different_org(self, email, requestor, requested_user):
+        """Verifies if an email belongs to a different organization as a member or invited member."""
+        # Check if user is a already member of a different organization than the requestor's org
+        requestor_org = UserPortfolioPermission.objects.filter(user=requestor).first().portfolio
+        existing_org_permission = UserPortfolioPermission.objects.filter(user=requested_user).first()
+        existing_org_invitation = PortfolioInvitation.objects.filter(email=email).first()
+
+        return (existing_org_permission and existing_org_permission.portfolio != requestor_org) or (
+            existing_org_invitation and existing_org_invitation.portfolio != requestor_org
+        )
+
+    def _send_domain_invitation_email(self, email: str, requestor: User, requested_user=None, add_success=True):
         """Performs the sending of the domain invitation email,
         does not make a domain information object
         email: string- email to send to
@@ -803,6 +818,13 @@ class DomainAddUserView(DomainFormBaseView):
             )
             return None
 
+        # Check is user is a member or invited member of a different org from this domain's org
+        if flag_is_active_for_user(requestor, "organization_feature") and self._is_member_of_different_org(
+            email, requestor, requested_user
+        ):
+            add_success = False
+            raise OutsideOrgMemberError
+
         # Check to see if an invite has already been sent
         try:
             invite = DomainInvitation.objects.get(email=email, domain=self.object)
@@ -859,16 +881,21 @@ class DomainAddUserView(DomainFormBaseView):
         Throws EmailSendingError."""
         requested_email = form.cleaned_data["email"]
         requestor = self.request.user
+        email_success = False
         # look up a user with that email
         try:
             requested_user = User.objects.get(email=requested_email)
         except User.DoesNotExist:
             # no matching user, go make an invitation
+            email_success = True
             return self._make_invitation(requested_email, requestor)
         else:
             # if user already exists then just send an email
             try:
-                self._send_domain_invitation_email(requested_email, requestor, add_success=False)
+                self._send_domain_invitation_email(
+                    requested_email, requestor, requested_user=requested_user, add_success=False
+                )
+                email_success = True
             except EmailSendingError:
                 logger.warn(
                     "Could not send email invitation (EmailSendingError)",
@@ -876,6 +903,17 @@ class DomainAddUserView(DomainFormBaseView):
                     exc_info=True,
                 )
                 messages.warning(self.request, "Could not send email invitation.")
+                email_success = True
+            except OutsideOrgMemberError:
+                logger.warn(
+                    "Could not send email. Can not invite member of a .gov organization to a different organization.",
+                    self.object,
+                    exc_info=True,
+                )
+                messages.error(
+                    self.request,
+                    f"{requested_email} is already a member of another .gov organization.",
+                )
             except Exception:
                 logger.warn(
                     "Could not send email invitation (Other Exception)",
@@ -883,17 +921,17 @@ class DomainAddUserView(DomainFormBaseView):
                     exc_info=True,
                 )
                 messages.warning(self.request, "Could not send email invitation.")
+        if email_success:
+            try:
+                UserDomainRole.objects.create(
+                    user=requested_user,
+                    domain=self.object,
+                    role=UserDomainRole.Roles.MANAGER,
+                )
+                messages.success(self.request, f"Added user {requested_email}.")
+            except IntegrityError:
+                messages.warning(self.request, f"{requested_email} is already a manager for this domain")
 
-        try:
-            UserDomainRole.objects.create(
-                user=requested_user,
-                domain=self.object,
-                role=UserDomainRole.Roles.MANAGER,
-            )
-        except IntegrityError:
-            messages.warning(self.request, f"{requested_email} is already a manager for this domain")
-        else:
-            messages.success(self.request, f"Added user {requested_email}.")
         return redirect(self.get_success_url())