There are script steps that can open a window to a FileMaker Layout as a modal dialog. That layout can have a global field with a drop down list of user names.
There is a Re-login script that can use the data entered by the user in fields on this modal dialog to re-log into the database.
The trick is to use file options to automatically open the file with a very minimal access account and password and then have OnFirstWIndowOpen perform a script that displays the window/dialog and uses Re-Login to log the user in. The script can be set to "run with full access privileges" so your "auto login" account can be severely restricted.