In looking at my manual it appears to be a tad complicated. Do you have your manual? If not, you need to register at the Sena site and then you can download it. I'm also not sure that everyone can talk at once even if you register a total of 4 headsets. From the manual it appears that you have to hit the jog dial a certain number of times to speak to someone in particular. They do have voice actuated mode but it looks like that is only to a particular headset at a time.
If for some reason you can't get hold of a manual, PM me with your email addy and I'll send it to you. I have the online version as well.
Also, Sena has forums for such questions at the Sena site. You have to be registered, though. Either they or a Sena user should be able to answer your question.