How to add an existing user to existing group?
using -a -G option, adds user to the supplemental group(s).

sudo usermod -a -G [groupname] [username]
How to change existing david’s primary group to www?

sudo usermod -g www [username]

 

How to verify user settings?

id [username]

 

source: http://www.cyberciti.biz/faq/ubuntu-add-user-to-group/