Just beside the "Message" box (the one you type a new message in) is a "Search in posts" box. You can type in any string you like to search all the posts for it. If you would like to search for a certain user, just type in their name. It'll return all of their posts plus any containing their name.

Edit: Ok, i just tried it and it doesn't find posts by that user, just posts with that user's name.