Some random notes about working with Linux user groups.
Add a Group
Add User to a Group
usermod -a -G <groupname> <username>
The user won't immediately be found in the group. In order to make this group active, he or she must log out and log back in again. If it is daemon, you can stop and start the process to load in the new settings:
service my_service stop service my_service start
See also http://serverfault.com/a/98917
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