i used to be apart ot a friend group like this, but i climbed the social circle with most of them to make myself a main part of the group. Anytime someone would make fun of my stutter, id retaliate with a "joke" about something they're insecure about. It helped them realize that maybe it wasn't alright to say that. I basically just actively tried to be the most confident in the room, and it worked. I recently left that group because i realized they were toxic to many other people and didn't want to be associated with them. But that was my experience!