addNewPlayers

Atomically admits a set of players. All ordinary constraints and every cancellable join event are checked before any match/player state is mutated, so a party is never split by a full instance, missing permission, another active match, or an API veto. A listener may see an uncancelled preflight event for an earlier member before a later member vetoes the batch; those events describe admission attempts and no player state has changed at that point.

Atomically admits players while an external authorization remains valid. The predicate is rechecked after cancellable API preflights so those callbacks cannot revive a cancelled party launch by changing and then restoring unrelated match state.