Hi someone joined my team then unjoined, now they are not on my boatlist anymore why?

That happens. Unless you and they are Facebook friends, the game connection between you two can be severed when they leave the club. Even when you two are FB friends, it is possible to lose that connection. I guess the reason is so that the club can't keep pursuing someone who has left. However, the player that left may rejoin the club if they search for the club and join/apply.