lesson learns, usermod command

on linux, probably unix too.
to modify a user a user information on shell you type

usermod

here's what i learn, to assignment a group to user, you type

usermod -G groupname username

here's the catch, it will remove all the other group the user is in.

Another thing that I learn,is that to append, a group to a user, on shell, use the -a parameter.
So to append group to user, type:

usermod -aG groupname username

one more thing, always add the -a parameter, before -G, for some reason, -G assumes that -a is a group. so it is always

usermod -a -G groupname username
usermod -aG groupname username

but never

usermod -Ga groupname username
usermod -G -a groupname username

could be an ubuntu bug.