I would like to have an inline entity reference form that will optionally provide a write-in text form if the entity doesn't already exist. Here's the use case: I want to reference a faculty profile that is associated with a particular research project; but I want the option to also associate another person who does not have a profile on the site. In a corresponding view, if the associated person has a profile on the site, their name will link to their profile, otherwise it will simply appear without a link.
Use Entity reference module to insert the references of the existing users to the content type.
This project provides D7 versions of the 'node_reference' and 'user_reference' field types, that were part of the CCK package in D6, at functional parity with the D6 counterparts.
For the rest of the users, you could use a textfield.