However, that's about all we know. What's the "B" team's deal? We'll have to wait until next season to find out (if it's even revealed then!). For now, all we can do is speculate based on what we know about these characters. So here are a few guesses about what these three girls want and how they came to be aligned.
"A" team victims. One theory that's been batted around the internet for a while now is that the Liars aren't the only ones the "A" team has harassed. It's possible that these three girls have been targeted too, and have now teamed up to try to take the "A" team down.
N.A.T. connections. Melissa and Jenna were both connected to the N.A.T. club, so it's possible this partnership is a remnant of that. Perhaps they are trying to protect N.A.T. club secrets (secrets that the "A" team might have even gotten their hands on). (How does Sharna fit in? Who knows. Maybe she just really likes Jenna.)
Ali's death. Who killed Ali? Honestly, who in the world knows anymore. It's possible these girls were involved (perhaps as an extension of the N.A.T. connection, perhaps not), in which case their alliance might be about keeping their secret hidden.
Anti-Liars. Would Melissa really want to hurt her own sister? Maybe, if she's mad enough about Spencer stealing her boyfriends. And Jenna certainly has legitimate gripes against the girls. (Again, we're not sure how Sharna fits in. At this point, we feel like we barely know her at all.)
Part of the "A" team. Most fans seem to think the "B" team is separate from the "A" team, and we're inclined to agree. But it could be another arm of the greater "A" team whole.