Use this:usermod -a -G openmediavault-admin admin this will put admin into the openmediavault-admin group too.
The post id admin
root@Jarvis:~# usermod -a -G openmediavault-admin admin
root@Jarvis:~# id admin
uid=998(admin) gid=100(users) groups=100(users),995(openmediavault-admin)
root@Jarvis:~#