To add a placeholder user, you will need to import the person's first and last name into a User-type field. For instance, if you have a field called Assigned To, and want to create a placeholder user with my name, you would create a spreadsheet like the one attached.
Then, you would go to the Home page of the app > Import/Export > Import into a table from a file, and choose your file.
When you run the import, QuickBase will create one blank record, with the placeholder user listed in the User field. You can go ahead and delete that record. Then if you go to the Users list you'll see the placeholder user there.