Group chat was a major feature got missed in Gmail Chat compare to Yahoo!. Now Google brings this feature up, checkout following screen shots:
1) Start conversation with anyone of your buddies & then click Options–>Group Chat to invite more people into this chat:
2) Invite others to your chat:
3) All done you can add more people into this chat any time:
This feature is only available on integrated chat box inside Gmail & not available on Gtalk desktop IM. We may soon expect this from Google.