I have added some custom fields to my D8 site's user profiles. Would it be possible to add users to a entity reference field on a node using one of these? For example phone number, as that is one of the fields. Or am I stuck referring to only the users username?
You can use any field to lookup a user in a entity reference field. You just need 2 things: an entity reference View & a configured entity reference field.
Through the Views UI, you can add a User view with an Entity Reference Display. Add any fields to the View that you'll want to search on. Once your do, the Settings link next to Format:Entity Reference list will popup a modal that will let select your Search Fields.
After you've created your view, add your node's entity reference field. Set the Reference method to Views: Filter by an entity reference view. Where View used to select the entities is set to the view you created in step #1. Once the new field config is saved, you'll be able to search for any users based of the text values in the fields selected in Search Fields.