So I think there would be resistance members who don't trust Finn or want revenge on the first order so they use him to take out their anger. And on his first mission with a team it's no one relate to poe or his circle of friends. And of course something goes wrong and the team sells Finn out to the first order thinking they'll be safe.

The first order takes them as well and they see how horrible Finn is treated and he even takes more punishment to protect them. Finn is able to break them out but is heavily injured and the team has to explain to Poe and the rest of the resistance what happened. This event immediately clears any doubters to Finn's loyalty and the team basically become his fiercest supporters.

Just let Finn suffer a bit and get all the love afterward :)