The clan structure is used by most of the features of the API.Clans interface. Its returned by GetClans and similar methods and is an input for most methods.
The Clan structure is interchangable with CSteamID and ulong
The JoinChat method will attempt to join the user to clan's chat room. The resulting callback will return a ClanChatRoom object indicating the state, providing access to the chat and being of use in other chat related methods.