Rulings

Hellraiser Goblin's ability also affects itself. If it enters before combat, it will have to attack that combat if able.
If, during your declare attackers step, a creature you control is tapped or is affected by a spell or ability that says it can't attack, then it doesn't attack. If there's a cost associated with having a creature attack, you aren't forced to pay that cost, so it doesn't have to attack in that case either.