It's not that you need to be using a .Mac account, it's that you need three people all using iChat on 10.4 with a fast enough computer and fast enough connection between the computers.
If you only have the two of you on a Mac, then you won't get to see it. You would need a third iChat user. All of them can be AIM accounts, though.
If you only know two iChat users with fast enough computers, you won't get to use it. In order to even participate in a multi-person iChat, you need a 1 GHz G4, a dual 800 MHz G4, or any G5 or Intel. In order to INITIATE the chat, you need a dual 1 GHz G4, or any G5 or Intel.
If you only know two people with fast enough internet connections (it requires 128 kbps upload speed from EVERY user, and the one starting the chat needs 384 kbps upload, which is faster than lots of DSL connections provide.)
And those are for the absolute minimum, crappy multi-person video chat. If you want the highest-resolution version, you need a dual G5 (dual 2.0 GHz to host) or dual 1.83 GHz Intel, and a 500 kbps upload speed to participate, 1500 kbps upload speed to host!