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.