Ah.. couple things going on, I think.
First, you have "setLeader" and the command is "selectLeader."
And s1 is the variable that contains the unit that you want to be the leader? Try aPGrp selectLeader s1 instead.
Second, the "join" command, I believe, needs the unit in the array, which I believe you have as the player aP, then the group - not the leader. So, I think the correct syntax would be [aP] join aPGrp.
Oh, the stuff I posted should cycle the "player" through all the units in the player group.
Edit:
In re-looking at it, does the player start off in the group, or on his own? Or is this all spawned?