diff --git a/config/locales/server.en.yml b/config/locales/server.en.yml
index b7b1613..6dd4b54 100644
--- a/config/locales/server.en.yml
+++ b/config/locales/server.en.yml
@@ -10,6 +10,7 @@ en:
     unassign_on_group_archive: "When a message is archived by a group, unassign message (reassign if moved back to inbox)"
     unassign_on_close: "When a topic is closed unassign topic"
     reassign_on_open: "When a topic is opened reassign previously assigned users/groups"
+    invite_on_assign: "When a personal message is assigned to a user or group, invite them to the PM if they are not already a participant."
     assign_mailer: "When to send notification email for assignments"
     remind_assigns: "Remind users about pending assigns."
     remind_assigns_frequency: "Frequency for reminding users about assigned topics."
diff --git a/config/settings.yml b/config/settings.yml
index ee2f55e..421fc9b 100644
--- a/config/settings.yml
+++ b/config/settings.yml
@@ -12,6 +12,7 @@ plugins:
   unassign_on_close: false
   unassign_on_group_archive: false
   reassign_on_open: false
+  invite_on_assign: false
   assigns_user_url_path:
     client: true
     default: "/u/{username}/activity/assigned"
diff --git a/lib/assigner.rb b/lib/assigner.rb
index 0bddf59..48c1a99 100644
--- a/lib/assigner.rb
+++ b/lib/assigner.rb
@@ -258,6 +258,21 @@ class ::Assigner
   def assign(assign_to, note: nil, skip_small_action_post: false, status: nil)
     assigned_to_type = assign_to.is_a?(User) ? "User" : "Group"
 
+    if topic.private_message? && SiteSetting.invite_on_assign
+      guardian = Guardian.new(@assigned_by)
+      if assigned_to_type == "Group"
+        unless topic.topic_allowed_groups.exists?(group_id: assign_to.id)
+          guardian.ensure_can_invite_group_to_private_message!(assign_to, topic)
+          topic.invite_group(@assigned_by, assign_to)
+        end
+      else
+        unless topic.topic_allowed_users.exists?(user_id: assign_to.id)
+          guardian.ensure_can_invite_to!(topic)
+          topic.invite(@assigned_by, assign_to.username)
+        end
+      end
+    end
+
     forbidden_reason =
       forbidden_reasons(assign_to: assign_to, type: assigned_to_type, note: note, status: status)
     return { success: false, reason: forbidden_reason } if forbidden_reason
diff --git a/spec/lib/assigner_spec.rb b/spec/lib/assigner_spec.rb
index b9debb4..06813f7 100644
--- a/spec/lib/assigner_spec.rb
+++ b/spec/lib/assigner_spec.rb
@@ -360,7 +360,7 @@ RSpec.describe Assigner do
     it "triggers error for incorrect type" do
       expect do
         described_class.new(secure_category, moderator).assign(moderator)
-      end.to raise_error(Discourse::InvalidAccess)
+      end.to raise_error(Discourse::InvalidParameters)
     end
 
     describe "updating notes" do
@@ -708,6 +708,45 @@ RSpec.describe Assigner do
     end
   end
 
+  describe "invite_on_assign" do
+    let(:admin) { Fabricate(:admin) }
+    let(:topic) { Fabricate(:private_message_topic) }
+    let(:post) { Fabricate(:post, topic: topic) }
+    let(:assigner) { described_class.new(topic, admin) }
+
+    before do
+      SiteSetting.invite_on_assign = true
+      post
+    end
+
+    it "invites user to the PM" do
+      user = Fabricate(:user)
+      assigner.assign(user)
+      expect(topic.allowed_users).to include(user)
+    end
+
+    it "invites group to the PM" do
+      group =
+        Fabricate(
+          :group,
+          assignable_level: Group::ALIAS_LEVELS[:only_admins],
+          messageable_level: Group::ALIAS_LEVELS[:only_admins],
+        )
+      assigner.assign(group)
+      expect(topic.allowed_groups).to include(group)
+    end
+
+    it "doesn't invite group to the PM if it's not messageable" do
+      group =
+        Fabricate(
+          :group,
+          assignable_level: Group::ALIAS_LEVELS[:only_admins],
+          messageable_level: Group::ALIAS_LEVELS[:nobody],
+        )
+      expect { assigner.assign(group) }.to raise_error(Discourse::InvalidAccess)
+    end
+  end
+
   describe "assign_emailer" do
     let(:post) { Fabricate(:post) }
     let(:topic) { post.topic }