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 album "Christmas 2010" album then "jack" will be able to see "Christmas 2010".
To find out more about user groups, see the dedicated article in our online documentation: User groups.