[root@localhost /]# groupadd <groupname>
If u wanna use group quota, the users should have same primary group. Normally, primary group of "user1" is "user1" itself.
If u wanna use groups w.r.t file permissions, secondary group is used.
To add a new user "joe", n to make "mygroup" as his primary group,
[root@localhost /]# useradd -g mygroup joe
To add a new user "mary", n to make "mygroup" as her secondary group,
[root@localhost /]# useradd -G mygroup mary