From Snomhomish County:

The ability to fetch user objects (or a list of user IDs) via either User List ID, and/or the rules of a User List.

I was able to get the User List object, but it only contains the rules. (I think that this is a bit of a misnomer, since this “User List” is actually a filter object, but w/e. Naming things is hard, but giving me a way to fetch the user objects via a User List isn’t!)