Thank you for your post.
Get ( AccountName )
This functions returns the name of the authenticated account being used by the current user of the database file. If a user is using the FileMaker Pro guest account, then [Guest] will be returned. A sample script may be:
If [ Get ( AccountName ) = "[Guest]" ]
Go to Layout [ "Invoice Details Web" ]
Go to Layout [ "Invoice Detail IT Staff" ]
Let me know if you need additional clarification.