How does iChat determine if you are capable of joining or hosting a multiperson chat?
When iChat is opened, it performs a series of tests to determine if multiperson chats are supported with available bandwidth. iChat checks to see if you're on the same network you regularly use. If not, it performs bandwidth detection to see if network bandwidth is fast enough to support a multi-way connection. If the test succeeds, then multi-way chat is enabled.
Tip: You can also use the iChat Connection Doctor in Mac OS X v10.5 and iChat 4 or later. Choose Connection Doctor from the iChat Video menu, then choose "Capabilities" from the Show pop-up menu. For supported features, a green checkmark appears. For others, a red X appears along with a brief explanation; for example if you can't host a multiperson video chat, a red X and "(No Camera)" may appear next to "Host Multiperson Video". Other explanations you may see include "(Insufficent CPU)", "(Slow Network)", and "(No Microphone)". If your Mac is capable of one-way only video or audio chats, "(One-Way Only)" will appear next to the type of chat.
Things to try if multiperson chat isn't enabled but system requirements are met
The network connection may not currently support the minimum bandwidth requirements. Work with your ISP or system administrator to examine the situation.
A firewall may be blocking iChat's ability to complete the test--see this article. If a firewall blocks the bandwidth test, iChat will check the QuickTime streaming settings in QuickTime preferences. If it has been manually set to 256 kbps DSL/Cable or lower, multi-way will be disabled. In some cases, if this value is set to Automatic, multiway may also be disabled. You should set this value to one that matches your network connection.