Change <user_name> with your actual system user name and <group_name> with named of group to which you want to add user.
The following command will will add user jack to mygrp group.
# usermod -aG mygrp jack
Remove User from Secondary Group
The following command will remove user from secondary group. Please replace <user_name> and <group_name> with appropriate values.
$ sudo gpasswd -d <user_name> <group_name>