You know I recently got back into persona (last played 3 fes like 7 years ago) but this is a genuine question, if someone did say no to joining sees what exactly would happen? Iirc shinjiro wasn't allowed to leave port island (unless I'm misremembering which I probably am) would've loved to see someone just go "yeah no imma do my own thing"
I don't think it's been established what would happen in that case. I don't think they'd be so strict as to do anything about it in such a case, since the ones who had power in that regard were Ikutsuki, Mitsuru, and to an extent Akihiko. They do come off as somewhat overbearing about it (which is why Yukari wasn't too happy about how much they were trying to recruit Fuuka), but I don't think there'd be any serious repercussions. Shinjiro left SEES and was repeatedly asked back into the group by Akihiko during the events of the game and said no almost every time. Akihiko was just stubborn about him specifically but he mostly was just like "I'll try again next time".
It also just so happened that Makoto, Junpei, and Fuuka had nowhere to go and the latter two wanted somewhere to belong so they readily accepted joining SEES. A lot of Persona Users became Persona Users because they at a subconscious level confronted and accepted their Shadow, embodying the things they fear and dislike about themselves and don't want to admit, even if not as explicitly as in P4, and in doing so gained a distinctly strong resolve, and SEES became a means for them to act on that. It shows when just about everyone who ever did join had their own reasons for wanting to join the fight.