will list all the groups, including ones used only by the system. Just typing will list all the groups of which the current user is a member. To add a user use To add a user to a specific group use
Code:
sudo usermod -G name-of-group -a 'name-of-user'
For more information on all these -- and it's really a good idea to make sure you know what you're doing before doing system-level commands -- you can type or For instance "man useradd".