FIX: Don't raise an error if you try to assign a group that exists

spec/controllers/admin/users_controller_spec.rb

@@ -37,7 +37,6 @@ describe Admin::UsersController do
           expect(UserHistory.where(action: UserHistory.actions[:check_email], acting_user_id: @user.id).count).to eq(0)
 
           xhr :get, :index, show_emails: "true"
-          data = ::JSON.parse(response.body)
 
           expect(UserHistory.where(action: UserHistory.actions[:check_email], acting_user_id: @user.id).count).to eq(1)
         end
@@ -173,6 +172,22 @@ describe Admin::UsersController do
       end
     end
 
+    context '.add_group' do
+      let(:user) { Fabricate(:user) }
+      let(:group) { Fabricate(:group) }
+
+      it 'adds the user to the group' do
+        xhr :post, :add_group, group_id: group.id, user_id: user.id
+        expect(response).to be_success
+
+        expect(GroupUser.where(user_id: user.id, group_id: group.id).exists?).to eq(true)
+
+        # Doing it again doesn't raise an error
+        xhr :post, :add_group, group_id: group.id, user_id: user.id
+        expect(response).to be_success
+      end
+    end
+
     context '.primary_group' do
       before do
         @another_user = Fabricate(:coding_horror)