This sounds like a similar situation with some of my family. Individually they are OK but as a group they revert to to their childhood personas and can be quite unpleasant and hurtful to be around.
It sounds like you’re prioritizing quantity of friends.
it sounds to me like OP wasn't a participant in this group (and is only after finding out about what happened), though I could be misinterpreting it
Also it wasn't like he was just bullied and shunned, he hung out with those guys too. I even did a road trip with that group. So it was a complicated thing that probably also evolved after school ended.
I don't think I ever thought about quantity of friends as a thing. It's not like it's binary whether someone is your friend.
A lot of people from tiny country/rural towns I think end up in this situation, because we often end up fighting the "Crabs in a Bucket" phenomenon when trying to be better humans, and leaving can be easier. Whilst I do think about people from my childhood and that town, it feels like an entirely different life to me.
Basically I'm the guy who keeps in touch with everyone. I have a chat group with guys I met at age 4. Everyone who went to school with me asks me "where is X now" and I'll tell them I last spoke to them not long ago. I talk to my old teachers, across the spectrum from 1st grade to high school. People from various jobs I did 20 years ago, I still know.
So I'm rounding up people for a high school signal group, and even those guys who never say anything to anyone will join it because it's memes and news articles we can comment on. Kinda nice to have all your old buddies there, even with lurkers bring a thing on private groups.
But this one guy, I know him pretty well, been to his house many times growing up, visited his parents, went to each others weddings, etc... Doesn't want to join.
It's just too painful. He feels tormented by some of the other guys in the group. Apparently they tried this some time ago and the chat descended into bullying him like we were in school, except now we're grown up everyone thinks it doesn't hurt anymore.
I really wish there was a solution, but I think it won't happen. Sometimes relationships go off the rails and never come back. I'll still go see my buddy next time I'm near him, but he's cut off everyone else.