Your database may have gotten confused. First of all create a backup of it via the GUI and store it somewhere safe.
You can try compacting it with the following:
http://www.cisco.com/univercd/cc/td/doc/product/access/acs_soft/csacs4nt/acs33/user/ae.htm#wp364964
This will sometimes fix the issue. If it doesn't, run csutil just with the -d option to dump it out to a text file. This file will be quite large, you need to look through this to find the group you're interested in. Let's say it's Group 2 under the GUI (note that you may have changed the name, but it will still be referenced within this file as group2 or profile2 in most cases).
So, if it's Group 2 that's showing the wrong number of users, look for the following line in the file:
#-----------------------------------------------------------------------------
Name : ###profile2
Everything below this line, up till the next line of dashes, is Group 2's configuration. Under here there's a line as follows:
User Count : 5
This is the number that appears as the user count within this group in the GUI. Change this number to the correct number, save the file and exit.
Now you need to import this file back into your database, so run the "csutil -n -l" now and the databse will be created afresh and then imported using the dump.txt file you've just modified.
