Do you mean raising an Event that many users can Observe?
If so, it can't be done with the Seam event model. You'll need to look into messaging eg PubSub or hack something around the Database.
Exactly what I meant.
Anyway I will find a way, or create a new one :-)
If you haven't started your application yet, you might want to try ICEfaces and use server-push to achieve this. There are a couple of examples if you are interested.