How do I add a custom field to allow the selection of a user in the system? Sort of similar to the Owner field of portfolio items or user stories.
Unfortunately, there isn't a custom field as what you're seeking, Owner field of portfolio items/user stories is a Rally built-in field.
I can certainly understand how an improvement of this functionality is beneficial and would like to encourage you to promote this idea in Rally Ideas (https://ideas.rallydev.com) and provide any discussion items that you think might be valuable.I can find one of them is similar with your one. You can vote this idea as well.
Thanks for bringing up Idea Manager! There's an idea that's a closer match at D519. If you subscribe to that idea, you'll get the latest updates as we investigate it.
Thanks. I added my vote to that idea. Now, it does not appear this will get addressed soon. Is adding such a user custom field something that can be done thru the custom app API?