It seems to me this would be a fair solution:
If Affiliate A signs up a NEW player to Casino 1 then he gets credit at casino 1 and all future cross promotion that GP does.
If Affiliate B signs up an EXISTING player to Casino 3 then he gets credit only at casino 3.
That way, the FIRST affiliate to bring the NEW PLAYER to the GP group gets credit for all cross promotion from the GP group. The only way this first affiliate can loose a potential commission is if another affiliate brings that player to another casino.
Make sense?