If you installed FileMaker Server, you can find the current user names accessing each file listed in the Server Console.
With FileMaker Pro as the Host, you'd need to add a table and code to log the name of each user (or their account names) each time they open the file. You can use File Options to select a script that will run when the file opens. The script can use Get ( UserName ) and/or Get ( AccountName ) to enter the name(s) and the date/time they accessed the file. A similar script set to run when the file closes, can then log them back out in this same table. Please note that there is no way to log out a user if their copy of FileMaker crashes or they are otherwise forced out of the hosted file.