How to get a list of Users as values for a select list field?
You will want to install a module that allows you to reference a user. A popular choice is entityreference. 

With Entity Reference you can create a view of User's with a display type of Entity Reference. You can then add a filter to your view to only show users with the role that you are interested in.

Then you can setup a new field on your content type of the type, reference. Within that fields settings you can specify your view to be used as the mode for "Entity Selection".

You only need to setup the view if you want to limit the users that are selectable. If any user would be valid for your field, Entity Reference has some built in modes for entity selection that makes the process even easier.