in your domain model create a reference to the user entity you're using, add that reference to your form.
A drop down box will display values from either an association or an enumeration. In your case, you will be displaying the values of an association between users and another entity. Simply create the association between your entity and the System.Users entity or any specializations of that entity. Then on a web form, create a dataview of your entity, put a reference selector in the dataview and attach it to the username field into the dataview. What you'll see when you view this form is a list of all users from the entity you associated to. Selecting one of these users will set the value of the reference.