As you have figured out, Google+ Chat and Google Chat are the same basic feature. The big difference is that because of the Circles of connections in G+, it's much easier to limit your availability to a subset of your friends and colleagues. Very helpful and it certainly helps me avoid banal or annoying interruptions from people wanting to ask me tech questions. That's why I have that big red "Ask Dave" button on the right! :-)

The mechanism for turning a two person chat into a multiple person chat is the same as it was in Google Chat, however, and the first step is to establish that two-person link.

Once you have that, you'll add additional people to the chat, even though (confusingly) as soon as you go from two people to three G+ Chat actually pops open a new chat window.

Let me show you...

First off, here's a two person chat between Chris and me:

As I've highlighted, to add an additional person to the chat, I need to click on the +person button. It pops up a small input box:

I start typing in the name of the person I want to add, Steve, and it immediately displays all the matching people from my address book:

At this point in the life of Google Plus, it doesn't show you by circle, but I expect that'll come. Instead, it's the standard icons: green means available, orange means busy or unavailable, and grey means offline. I'll add Steve even though it looks like he might well be busy by clicking on his name:

There ya go. A new G+ Chat window with the three of us able to chat to our heart's content.

Now if you want to spread the love a bit more with a larger discussion, definitely try a Google Plus "Hangout". More on that very, very soon...