One interesting (but admittedly only rudimentarily defined) aspect of Liberty's new People Service is the ability for users to tag the friends, colleagues, and family members (and any groups into which they are placed) that are members of their PS list.
Each friend or group Object can have multiple optional Tag elements, each with a ref attribute carrying a tag value (recommended to be in some established taxonomy like Flickr or Del.icio.us).
Tags are seen as an extra (to the friendly name) axis along which user's can categorize their friends and groups. As an example, if a user created two groups called 'Soccer Team' and 'Hockey Team', both groups could be tagged with 'sports' to make clear their shared relationship (as opposed to placing both in some extra hierarchal level through a 'Sports Teams' group).
Once both groups are tagged, access rights (e.g. allow any of my team-mates, irrespective of sport, to see this video) or group operations (e.g. send a party invite to all my team-mates) can be defined against the tag rather than the individual groups or individuals that have that tag.
We have much to learn here.