Before Slip, Zeroes and Nines, Finn was on another unit. But the First Order split them up because they worked too well together/grew attached. The transfer was what sparked Finn's first doubts about the Order. Post TFA Finn is enjoying his time at the Resistance Base, but feel that he needs to go back for his unit. He either leave his new friends behind or run into them on a mission.

+ Leia asking the stormtroopers why they are helping and they're just 'FN-2187 are helping you, that is good enough for us.'
++ Finn's Resistance friends pointing out that Finn's name is FINN not FN, 87 or FN-2187.
+++ Poe being jealous by the new people that are super close to Finn.