If you don't already know, gmail has had a feature called Suggest more recipients for a while now.
From the link above, "Gmail will suggest people you might want to include based on the groups of people you email most often. So if you always email your mom, dad, and sister together, and you start composing a message to your mom and dad, Gmail will suggest adding your sister. Enter at least two recipients and any suggestions will show up like this."
I am planning on extending this even for the chat conversation start page in the chat client that I'm working on. So, if you start entering JIDs of people you want to chat with, the client should automatically suggest more participants.
Thanks to Sandy for the providing the link to gmail blog!!
Update (02/01/2011): This feature is now implemented and part of the dev. versions.