User assigns the defect to another user. If both users are from the same group (Testers or Devs) or one of them is TL then there is no action. But if Tester group member tries to assign Defect to Dev group member, there should be a message "You can't assigne Defects to this user.".
Current user's group I can check using User.isInGroup(<groupname>) - but the <groupname> has to be precise. I don't know how to get it more flexible and reusable (for example in case when there will be more groups then Devs, Testers and Test Leaders). Another thing is how to get what groups member is a user with <username>. Of course I can grab <username> from Bug_Fields("BG_RESPONSIBLE").Value - but still, how to extract the group(s) name(s)?