users

I had to add some users to a remote site.
X traffic from there is way to slow to
use system-config-users.

I got around it by doing

for nam in user1 user2 user3 user4
do
   groupadd $nam
   useradd -g $nam $nam
   echo $nam | stdin -- passwd $nam
done

(Passwords were actually set to something else)

But now I realize they should be in group x
(I don't want to change their original group,
just add them to group x).  Normally I would
use system-config-users and add them but
the x traffic is too slow.  I am not sure how
to do it without X.  Is it simply a matter
of editing /etc/passwd and adding the
additional groups?  Or perhaps editing
/etc/group and adding ,user1,user2 ....
to the entry for group x?  Is there anything
else that would need to be done?