What HCM functionality is available for non HCM users (flexfields e.g.)

Summary:

In order to support various business needs and extensions we'd like to attach more information at the person/user level but are struggling when trying to get a straight answer as to what is available. Looking for suggestions for user level flexfields and national identifier.

Content (required):

We're looking to create a connection between users and paygroups and the natural way to do so would be to create a flexfield at the user level that validates against paygroups.

I haven't been able to locate user level flexfields, so I've tried setting up a flexfield at the per_person level. The flexfield does not show up in the "Users and Roles" form that is referenced in