To reproduce this bug:

1. Create User1 and User2 in seperate User groups
2. Create Action to send notification to User1
3. Execute created Action
4. Log in with User2
5. In Dashboard (Last 20 issues) or Monitoring (Events) if we click on Actions:1 column USER is empty, however if we click on ACK, in USER column we can see User1
6. If we open Event Details - in Recipients column there are no Username. Plus in the bottom error

    Undefined offset: 1 [tr_events.php:114 ? getActionMessages() in include/actions.inc.php:1116] 

If both users are in same usergroup nothing of this happens.