2014-07-18 : Although Goblin Rabblemaster doesn’t force itself to attack, if you control two of them, they’ll force each other to attack if able.

2014-07-18 : If, during your declare attackers step, a creature that must attack if able is tapped, is affected by a spell or ability that says it can’t attack, or hasn’t been under your 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, you’re not forced to pay that cost, so it doesn’t have to attack in that case either.

2014-07-18 : The number of attacking Goblins is counted as the last ability resolves, and the bonus is locked in at that time.