'-g' option of the roundup-server script is broken in
two ways:

first, it ignores it's argument.  instead, gid is set
to the primary group of the server user (set by
constant in the server script or by '-u' command line
option).

second, it assumes that setgid() is allowed to users of
group 0.  that's not so.  from setgid() manual: "The
setgid() function is permitted if the specified ID is
equal to the real group ID or the effective group ID of
the process, or if the effective user ID is that of the
super user."  that is, we must check for zero uid, not
for zero gid.

the attached patch should fix both these problems.

warning: the patch is not tested.