Groups are a logical collection of users and access. You can create as many groups as you need.  Users can also belong to unlimited groups.  The power of groups comes with the ability to add access, event roles and send notifications.  This is a great way to keep the group/team all on the same page. 

As an admin, you can even configure groups to allow users to join/leave on their own.  So if you have groups that you want to be just for notifications and use as a central message board, this is a great option.

To access your groups as an admin, login and go to Users and then click Groups.  Individual users will be able to see which groups they are in when they login on their user page.