Fillout doesn't currently support user fields. Your best bet would be to create a new table of your users in Airtable. Each record in your Users would have the user's full name as the primary field, and you can also add a User field there for Airtable's purposes. In your main table, you can add a linked record field which links to your Users table, and that linked record field can be used in Fillout.

Hope this helps!

— ScottWorld, Expert Airtable Consultant