Creatures that enter the battlefield (or noncreature permanents that become creatures) after the ability resolves are affected by the ability and must attack if able.

The controller of each creature still decides which player or planeswalker that creature attacks.

If, during a player’s declare attackers step, a creature is tapped, is affected by a spell or ability that says it can’t attack, or hasn’t been under that player’s control continuously since the turn began (and doesn’t have haste), then it doesn’t attack. If there’s a cost associated with having a creature attack, the player isn’t forced to pay that cost, so it doesn’t have to attack in that case either.