You can always try programs like Roger Wilco or TeamSound. However, conversations held in these programs are usually too confusing (any more than two people speaking at once makes it very difficult to follow).
A better idea would be to run something like a radio broadcast off the ShoutCast network. Then you could "accept callers" and let one or two people voice their opinions, while still allowing a large audience to listen in.
In fact, I think you can selectively mute people to achieve the same effect.
Run a tandem IRC conversation and people could request to have the floor.