When Tinybones Joins Up’s first ability resolves, the next target player in turn order (or, if it’s a target player’s turn, that player) chooses a card in hand without revealing it, then each other target player in turn order (if any) does the same. All chosen cards are then discarded at the same time.
When Tinybones Joins Up enters, any number of target players each discard a card.
Whenever a legendary creature you control enters, any number of target players each mill a card and lose 1 life.
