There is a user accounting tab in each users properties that shows you how much bandwidth that user has used and how much time was spent online.
Gatekeeper > Users tab > Expand out the users section > double click on a user > Check the Accounting tab.
As for bandwidth usage by user groups, this is not recorded in WinGate, however a logfile analyzer might be able to work this information out by adding the bandwidth usage of users from the same group together.
Some popular logfile analyzers are;
Proxy Inspector for WinGate:
http://www.advsoft.info/en/download/
Saw Mill:
http://www.sawmill.net/
Internet Access Monitor:
http://www.internetaccessmonitor.com/
I can only find the log for for my proxy and it only show the site visited by users not the traffic consumed.
There should be alot of entries that look like the following in the logfiles. These are recorded after a session finishes to sum up the amount of bandwidth used.
09/06/05 16:25:04 250.18.29.58 0000018770 Traffic 493 250 196 512 2s
According to the helpfile, the traffic represents "Bytes (sent to, received from, sent for, received for) client, and seconds online". I hope this helps.