They didn't join his group, they helped to found it. After they survived the Hydra attack together during Captain America, as shown in the flashback 2 episodes ago, they started this group as the new Shield. When they found out Coulson was recruiting for a new Shield Bobbi, Mack and Xena were sent to spy on Coulson to see if they could trust him and join him as one big team but decided they couldn't. As for why they would stay now that the leadership seems to have an anti powers agenda, Mack seemed to be anti powers himself after what happened to him in that temple and him saying that they needed to be protected from Skye when they found out she had powers. As for why Bobbi would stay, it's not easy walking away from something you started, especially if you just tossed your other options and your friendships/relationship in the bushes for this group, but she does seem more uncomfortable with the superpower cism than Mack so she may turn down the line.
I'd say it's 100% that Bobbi joins Team Coulson at some point