You've probably noticed the new button on The Switchboard applet: "conference". You can use this new feature to initiate a conference call with multiple users. Simply add each contact one at a time to the conference, but keep in mind that all of the conferencees must be in each others contact lists as well.
There is no limit to how many people can engage in a conference call, but in reality your hardware and network connection will limit you. If you exceed 5 participants you will probably have quality issues, as you hit your upload limit.
Let me know how many you can get going at once :).
You'll need ~4 KB/s up for each connected participant. If you switch on the low bandwidth mode this will drop to ~2.5 KB/s. I don't think 6 participants should be a problem if you have a decent broadband connection.
