fix: Default group.member_ids to an empty array

addons/api/addon/models/group.js

@@ -1,6 +1,7 @@
 import GeneratedGroupModel from '../generated/models/group';
 import { attr } from '@ember-data/model';
 import { computed } from '@ember/object';
+import { A } from '@ember/array';
 
 export default class GroupModel extends GeneratedGroupModel {
 
@@ -10,13 +11,14 @@ export default class GroupModel extends GeneratedGroupModel {
    * Members is read-only under normal circumstances.  But members can
    * be persisted via calls to `addMembers()` or `removeMembers()`.
    */
-  @attr({readOnly: true}) member_ids;
+  @attr({readOnly: true, defaultValue: () => A()}) member_ids;
 
   /**
    * An array of resolved user instances.
    * Unresolvable instances are excluded from the array.
    * @type {[UserModel]}
    */
+  // TODO: Use fragments array to read cross scope members.
   @computed('member_ids.[]')
   get members() {
     return this.member_ids

addons/api/tests/unit/models/group-test.js

@@ -130,4 +130,17 @@ module('Unit | Model | group', function(hooks) {
     const model = store.peekRecord('group', '123abc');
     await model.removeMember('3');
   });
+
+  test('it defaults `members_ids` to an empty array when model instance does not define it', function(assert) {
+    assert.expect(1);
+    const store = this.owner.lookup('service:store');
+    store.push({
+      data: {
+        id: 'group_123',
+        type: 'group'
+      }
+    });
+    const group = store.peekRecord('group', 'group_123');
+    assert.equal(group.member_ids.length, 0, 'Group has empty member_ids by default');
+  });
 });

ui/core/app/components/form/group/add-members/index.hbs

@@ -9,7 +9,7 @@
   >
 
     <form.actions
-      @submitText={{t "actions.add-host-sets"}}
+      @submitText={{t "actions.add-members"}}
       @cancelText={{t "actions.cancel"}} />
 
     <Rose::Table as |table|>