they have appeared out of nowhere on leader board,
No big deal....pointless exercise unless you get pleasure seeing your CC's name there. There are no benefits, no kudos, no skill and no reward....just a measure large clubs using passes.
Poaching as reprehensible as it is morally, is not a crime nor a breach of T&C's.
A player who is happy with their current club will not leave based on a random (or otherwise) invite.