Currently, User Fields can only be used in certain ways on forms. They will not work in Add or Edit Record mode, meaning that form completers will not be able to input or change the value in a User field on a form (if you need a substitute, try a Text - Multiple Choice field instead of a User field for the form).
User fields (and Lookup fields to a User field) can be used for View/ Read-only display of User field data already in Quickbase.
However, because of the way Quickbase encodes the User (or Lookup to User) field, to make a User field display the User Name instead of a Quickbase code on the View or PDF generated form, you need to create this additional Quickbase Formula - Text field:
In the Formula, replace [Employee] in the example above with your User or Lookup to User field. This new field will now display the User Name of the User field, so use this field on the form instead of the User (or Lookup to User) field directly.
NOTE: The List-User field type will not currently work in forms in any type of use, the Form Engine will NOT recognize this field type.