DI reader Aravindan is currently using a combination of Google Talk and the collaboration features of Google Docs for chatting with more than one GMail friends simultaneously in one browser window. He writes:
I always wondered why google does not have a text or voice conference feature in their google talk software, and I found that a handicap a few times. 
Yesterday, I had to collaborate on a Google Spreadsheet with 5 of my friends, and after setting up the file and inviting them, the discuss option was enabled in my window, and I would see all 5 of their email's listed. 

There was a shoutbox like feature embedded in which the 6 of us were able to conduct a discussion while editing the spreadsheet. Therefore, to have a live conference using google chat just follow these steps in Google Docs & Spreadsheets : Create a dummy document -> invite friends as collaborators -> click on "Discuss" option near top right -> start conferencing.
That's innovative but there's a more simple method for group chatting with more than one GMail contacts inside the same GTalk window.

First, open the talkgadget client webpage (or load Google Talk in Sidebar) and sign in using your Google Account / GMail credentials. Next, initiate a conversation with any of your Google Talk contacts who's online and then click the "Group Chat" button.

That's it. You just have to invite your other Google Talk friends to this private Google conference room. And there's no limit on the maximum number of participants that can join your Gtalk conference session.

Like other internet chat rooms, Google Talk will also announce each new participant as they join with a message in the chat window. And you can also use tons of interesting smileys (emoticons) in your group chats which are otherwise missing in the desktop Google Talk software.