That's been done a few times with Tinc (open source VPN) [1][2]. It's great until the group includes one bad apple. All it takes is one person with low impulse control, low opsec to bring the whole thing down. Some hacker groups did this some time ago.

[1] - https://www.tinc-vpn.org/

[2] - https://www.linode.com/docs/guides/how-to-set-up-tinc-peer-t...

That's just the technical problem. It can be solved.