Hi Greg,
What you request is not available/supported. If you look at the property fields on the user configurator you’ll notice that they have a white background as opposed to a pale yellow one.
All object properties on a white background are constant, read only, values. Another example is the page identifier property in the page object. So you are able to temporarily use a formula to get a value in them but the formula is not permanently stored in the field but just a constant value.
In the case of project users only you as developer can add or remove them and assign their access level. When this must change you must submit a project update.