When a user is a member of a group, and a ticket is assigned to that group, all members of that group are notified with an email.  This email can be turned off on a per user bases.  See the Users help for more info.  Users can also be members of multiple groups - which is perfect for those wearing multiple hats!

Below is the view from a ticket showing the assigned group.

Below is a view of the groups in action.  The (0) indicates the number of open tickets in each group.