Each creature's controller still chooses which player or planeswalker the creature attacks.

You can't attack yourself or your own planeswalkers to give Thantis +1/+1 counters, but Thantis appreciates the way you think.

If a creature can't attack for any reason (such as being tapped or having come under that player's control that turn), then it doesn't attack. If there's a cost associated with having it attack, the player isn't forced to pay that cost, so it doesn't have to attack in that case either.