There are two ways you can view the users and the groups that they belong to. The easy way is to just go to User Setup and click Find. This will list all of the users in the right hand pane and will show status and group membership. The second way is to do the following:
Open a command prompt and change to the c:\Program Files\CiscoSecure ACS v3.x\Utils
Enter the following commands
net stop csauth
csutil -u
net start csauth
This will create a file named users.txt which will contain all of the user names in the order they were entered and also what group they are in.