Create a custom field of type 'User'

I'd like to see a custom field of type 'user' that is a drop-down of the currently active users (maybe this could be filtered by group so that the list doesn't get too long?).

When we assign tasks, we also assign a person as the designated design partner and code reviewer. The easiest way to do this in Jira at the moment is to either manually create a list (alas, there must be a default for it) or to use a text field. It would be preferrable for our process to have a process whereby you could assign from a drop-down for this list.

I'm not sure yet how the workflow is setup, but it would be nice as well if there were some way to customize the workflow to assign to the reviewer after the developer has completed their work.