I don't think there's a way to do that as you would with something like Google Hangouts. You can start a group chat, and invite the users to the group. You could emulate a broadcast behavior with Jabber SDK in a web application. You'd just have to provide a way to select multiple contacts, and then write the application to send the message to each of them. But if you want them to reply for all to see, you'd still have to create a group chat.
