Linux - Add User to a Group

1. To add existing user to existing group:

$ usermod -G groupname username

or 

$ usermod -g GID username

2. To add existing user to existing supplementary groups

$ usermod -a -G groupname username

-a is switch for append

3. To add user to group when adding user

$ adduser --ingroup groupname username

4. To check which groups user is member of

$ groups username