I have players, who are in same group (e.g. invite to group) so they cannot damage each other which works (and see each other as a red dot on map which is great!).
However they would like to train in the arena and damage each other by leaving this group. I see no way to do this. 
I suggest, if you point reticule to player who is in your group, to change the invite to group option to 'remove from group' if those layers are already 'grouped'. If you select invite to group option, it only reads 'player is already a member of your group'
