FEATURE: group name is required for private topic invite

app/assets/javascripts/discourse/controllers/invite.js.es6

@@ -23,8 +23,9 @@ export default Discourse.ObjectController.extend(Discourse.ModalFunctionality, {
     if (this.get('saving')) return true;
     if (this.blank('email')) return true;
     if (!Discourse.Utilities.emailValid(this.get('email'))) return true;
+    if (this.get('isPrivateTopic') && this.blank('groupNames')) return true;
     return false;
-  }.property('email', 'saving'),
+  }.property('email', 'isPrivateTopic', 'groupNames', 'saving'),
 
   /**
     The current text for the invite button
@@ -46,6 +47,15 @@ export default Discourse.ObjectController.extend(Discourse.ModalFunctionality, {
     return this.get('model') !== Discourse.User.current();
   }.property('model'),
 
+  /**
+    Is Private Topic? (i.e. visible only to specific group members) 
+
+    @property isPrivateTopic
+  **/
+  isPrivateTopic: function() {    
+    return this.get('invitingToTopic') && this.get('model.category.read_restricted');
+  }.property('model'),
+
   /**
     Instructional text for the modal.
 
@@ -59,6 +69,19 @@ export default Discourse.ObjectController.extend(Discourse.ModalFunctionality, {
     }
   }.property('invitingToTopic'),
 
+  /**
+    Instructional text for the group selection.
+
+    @property groupInstructions
+  **/
+  groupInstructions: function() {
+    if (this.get('isPrivateTopic')) {
+      return I18n.t('topic.automatically_add_to_groups_required');
+    } else {
+      return I18n.t('topic.automatically_add_to_groups_optional');
+    }
+  }.property('isPrivateTopic'),
+
   /**
     The "success" text for when the invite was created.
 
@@ -76,6 +99,7 @@ export default Discourse.ObjectController.extend(Discourse.ModalFunctionality, {
   reset: function() {
     this.setProperties({
       email: null,
+      groupNames: null,
       error: false,
       saving: false,
       finished: false

app/assets/javascripts/discourse/templates/modal/invite.js.handlebars

@@ -14,7 +14,7 @@
     {{text-field value=email placeholderKey="topic.invite_reply.email_placeholder"}}
 
     {{#if isAdmin}}
-      <label>{{i18n topic.automatically_add_to_groups}}</label>
+      <label>{{{groupInstructions}}}</label>
       {{group-selector includeAuto=false groupNames=groupNames placeholderKey="topic.invite_private.group_name"}}
     {{/if}}
   {{/if}}

config/locales/client.en.yml

@@ -902,7 +902,8 @@ en:
         success_message: 'You successfully flagged this topic.'
 
       inviting: "Inviting..."
-      automatically_add_to_groups: "This invite also includes access to these groups: (optional, admin only)"
+      automatically_add_to_groups_optional: "This invite also includes access to these groups: (optional, admin only)"
+      automatically_add_to_groups_required: "This invite also includes access to these groups: (<b>Required</b>, admin only)"
 
       invite_private:
         title: 'Invite to Private Message'