You might look at groups and relational segments, both of which are subtyped from party.
Whether to extend parties or to store stuff separately from parties will depend on the type information you want to store and what you want to do with it.
For instance the photobook (extended user information, in reality, the name is an MIT-ism) doesn't extend user but rather maps a content item to each user and extends content revision to hold the actual information. This means photobook can 1) keep an audit trail of changes (which Sloan/MIT wanted) and 2) use the automatic form generation features of the CR/CMS)
I think it maps using the cr_item<->object mapping functionality of the content repository.
Groups and relational segments on the other hand extend party with a minimal amount of information and no audit trail of changes is necessary so simple subtyping is used.
