According to the character list...
Abram/Abraham is from the house of Montague, and Gregory (alongside Sampson) is a Capulet servant (they're the pair that provokes a fight with Montague in the beginning of the play)
Balthasar is the loyal servant that brings news of Juliet's "death" to Romeo, so he's in Montague.
And of course, Romeo is the Montague heir/"prince"