have you looked through the "User Level Authentication" Tutorial that is available in the solution Recipe Section of the SecurityAssist support page?
A group is a collection of values that a selected variable can equal.
In the UserLevel authentication tutorial, there is a Userlevel session variable being set on login.
The variable can have a value of 1, 2 or 3.
the tutorial crates a group with members 2 and 3.
then creates a rule that compares the UserLevel Session variable to see if the it belongs in that group.
Look through the tutorial, it should clarify Access Groups for you.