Google has added group chat functionality to GTalk client. Using this, you can start a conference and chat with multiple people at once.
Once you initiate the conversation with your contact, you can invite others thru’ Options menu (bottom left of your chat window)
In order to use the same functionality in your desktop GTalk client, download the latest version from here.
As far as IM market share is concerned, Y! still leads the pack (and Google is still playing a catch-up game). But Google is smartly pushing it’s IM client in all the apps (like Gmail, Google apps) and the future might look completely different!
By the way, which IM do you use?