Add a existing user to existing group (usermod)

If we want to add the existing user pron to the existing group vboxusers, we can execute this command:

$ usermod -Ga vboxusers pron

Is neccessary to put -a, otherwise other groups will be erased. To see changes type:

$ id

Resources
http://www.cyberciti.biz/faq/howto-linux-add-user-to-group/