"civilian" createunit [_pos,(group player),"_man = this;[this] join grpnull "]
Your scripting is fine, however I think I have found a bug in the game...
With this exzact same scripting, I can do "civilian" createunit [_pos,(group player),"man = this;man setdammage 1"] and it works, however with grpnull, the unit refuses to leave his group. This seems to be a bug.