How to add user to group in Linux

• To add an existing user JoeUser to an existing group FooGroup in Linux:

$ sudo adduser JoeUser FooGroup

• This command can be used, for example, to add an user to the sudo group and make it a sudoer:

$ sudo adduser JoeUser sudo

• If the adduser program is not available, you can achieve the same with the classic usermod program:

$ sudo usermod -a -G FooGroup JoeUser

Tried with: Ubuntu 16.04