There is no way for miniExtensions to know all the Airtable users that can be used in the user field. Our forms can only suggest users that are saved in your existing records.
The workaround is creating a dummy record in Airtable where all your users are saved in the user field.
It may take some for the Airtable users to show up in the drop-down selection since we cache it every 30 minutes. If this doesn't work, please follow this article: How to force update changes in Airtable base structure in miniExtensions?