[unitName] join GrpNull
grpLeader was just whatever unit name you typed in the leader's name field.
;D Nevermind, try this instead
if you're having trouble :
name yer original leader ROB and put this in his INIT field :
robsters = units group this
And The soon-to-be leader should be named BOB.
Then your trigger could go :
"[_x] join bob" foreach robsters; [rob] join grpNull