From iChat AV Help:
Topic: Hosting a Multiperson Video Conference
iChat AV for Mac OS X v10.4 supports video chatting with up to three buddies at once. To do this, one buddy "hosts" the video conference, and the others participate. Everyone in the video conference sees and hears the other participants.
You can invite three people to a conference at once, or let participants join and leave as the conference goes along.
To see if your buddy supports multiperson video conferencing, check the icon next to your buddy's picture. If the icon is multiple videocamera icons, you can invite your buddy to a multiperson video conference.
If the icon is multiple audio icons, you can invite your buddy to participate in the video conference using audio only. Your buddy joins the conference and is represented by their buddy icon and a sound-level meter, so you can tell when that buddy is talking.
To start a multiperson video conference:
Select the buddies in your Buddy List you want to invite to the video conference. Press and hold the Command key to select more than one buddy. Click the videocamera button at the bottom of your Buddy List.
You can invite three people to a conference at once, or invite participants as the conference goes along.
To host a multiperson video conference, you need:
An iMac G5, Power Mac G5, or fast dual Power Mac G4 (Any computer with a G5 processor or dual G4 processors that are each 1GHz or faster)
A fast broadband Internet connection (384 Kbps or faster).
Mac OS X 10.4 or later
To join a multiperson video conference, you need:
A Mac mini, iMac G5, Power Mac G5, fast iMac G4, fast eMac, fast PowerBook G4, fast iBook G4, or fast Power Mac G4 (Any computer with a G5 processor, a single G4 processor that's 1GHz or faster, or dual G4 processors that are each 800 MHz or faster).
A broadband Internet connection (100 Kbps or faster).
Mac OS X 10.4 or later
To see which processors are in your computer, choose Apple > About This Mac. The dialog displays the number of processors, their type, and their speed.
To find out the speed of your Internet connection, contact your Internet provider or refer to the literature they provided you. Note that the speed listed in the provider's literature is a maximum speed and that the actual speed may be lower.