Many uses for having an OpenID provider on your wiki are to provide an authentication source that can be used for other sites.
Often these may apply only to a subset of users, for which user groups are usually a pretty good way to handle it. A foreign site could, for instance, limit authentication to users on a given site in a given group.
Using the OpenID Teams extension, the relying party can ask whether that group membership is valid at auth time, and get (at least part of) the list of available groups back.
I implemented the relying party side for StatusNet's OpenID plugin a while back for integration with LaunchPad which uses this extension; it shouldn't be too hard in theory to stick in on the provider side here.