User membership in groups defined as +mygroup aren't looked up
group membership in /etc/groups. This was acknowledged by Volker Lendecke,
and suggested that group might be picked up in network groups, but this directly
contradicts documentation which explicitly states
"A name starting with '+' is interpreted only by looking in the UNIX group database."
So I guess there's a bug either in docs or the implementation.
Maybe it's a little confusing but technically the Unix group database
is serviced by NSS and is not strictly the /etc/group file.
I'll make the docs clearer.