I am not using Turnkey and had set-up 2 users using
useradd name -g mc.
This does not add those users to the group like a supplementary group. I suspect the default mc user in the other thread is using mc as its login group too.
mounts.py, getgrnam(group) returns an empty array of members in this situation
usermod name -G mc adds the user to the group list in
/etc/group and creating a server works.