While trying to group chat with a couple of friends, I discovered two things. 1) It's not possible on Gmail alone. 2) My Vista PC doesn't appear to have come with a Microsoft IM program. Instead of pre-installing it, Microsoft put "Windows Live Messenger Download" on my Programs menu. It looks a clever dodge of antitrust regulators' demands that Microsoft not abuse its operating system dominance by putting all of its software on the OS. Instead, customers have to take the step of downloading it, just as they have to download competitors' software. Microsoft has retained only the wee advantage of having their software be just a little more accessible than others'.
At least this does make a bit more meaningful the standard end user license agreement greeting "THANK YOU FOR CHOOSING MICROSOFT!"