Why do you need an enumeration? Create an entity with just one field and reference it.
I recommend to use the auto-complete widget (https://appstore.home.mendix.com/link/app/2695/) and map it to an entity with your users
Hi Aftab,
Make sure you have an association with the object containing the users. The population of the dropdown can then be done automatically or be done via a microflow.
Hope this helps.
KR