Re: max n of groups per user?
to not answer your question...
how about assigning group owner ship of the home directories to and
drwxrwxr-x <user> admin /home/<user>
If there's to be collaboration among these users, I'd make the
directories world readable, or instead of an admin group have an all
users group and mod 750.
I have >1,000 users and don't have difficulty with my UID not having
write access to their directory, if needed `sudo -u <user> bash` does
the trick nicely.