Groups are a convenient way to set permissions for a list of users.

Create groups in Administration » Users » Groups and associate users to this group in Administration » Users » Manage.

A user can belong to several groups. Authorization takes precedence over interdiction : if user "jack" belongs to groups "family" and "friends", and that only group "family" can see category "Christmas 2003" category then "jack" will be able to see "Christmas 2003".