I believe there's no reporting in Serv-U. Although you can check the Domain and go to domain details. You can also go to the individual user and then go to general information. I believe the very last tab
The best you can do really is to get a list of most recently logged in users and the only way to do this is..
Domain > Users > Click 'Last Logged in' heading to order the list.
...and then screenshot it! Not ideal at all but its an option.