I was very proud to be accepted into The Home Guard clan for Day of Defeat:Source yesterday.
Emergent Functionality One of the things that interested me is that this is a game community that is organised without there being specific functions in Steam to assist it. It is a convention to append your clan sig after your character name, I am now 'Zendog |HG'. When someone is an applicant then they use lower case, e.g. 'Zendog |hg', and post to a special forum thread with some personal details. They will then be monitored by the senior clan members on the servers for a couple of weeks to see if they meet the appropriate playing style (in the case of The Home Guard this is that you 'have fun and play fair').
Now, Steam could add some new forms into their control panel to help clans manage memberships, or they could enhance the system by parsing the naming convention. See machine tags in Flickr for an example of emergent functionality.