There is no script that shows this info. If you have server, you can monitor the logged in users there. A work around may be to keep a seperate log table that tracks when a user opens the file and exits the file using opening and closing scripts.
If your files are hosted by a version of filemaker server, it may be easier to simply use server admin to see who's logged in.
Before a script in filemaker can show this, you have to write scripts that use that function to record the account names as people log in and out of the data base by making entries in a table. This can be done with scripts you set to run when the file is opened and closed. (See File Options.)
Then your script to see who's logged in just displays data from this table. One issue that can mess this up: If a person's copy of filemaker crashes or their connection is lost, there's no way for the script to log the fact that they are no longer logged into the database.