Sometimes you might be waiting for a certain person to come online, and you don't want that occasion to get lost in the noise. To get an obtrusive message when that happens, type M-x jabber-watch-add and select the person in question. You can enter a comment, to remember why you added the watch.
You will get a message whenever that person goes from offline to online. jabber.el will remember this for the rest of your Emacs session (it's not saved to disk, though), but if you want to get rid of it, type M-x jabber-watch-remove.