How do I list all users and their email addresses within an application?

How do I list all users and their email addresses within an application?

The application I created is a system where by users submit issues.  An admin will then assign the issue to a particular user (not necessarily the one who submitted it).   Obviously I need to create the users in the Setting page and allow them access to the parts of the app that are relevant to them.   

My question is this:   Is it possible to create on a form a dropdown box that will show the available users or do I need to create the user a second time within a table (ie.  Name, Email, etc...)?

Thanks,
Scott