Remember that these PTR bosses are open for testing purposes -- don't forget to leave your feedback for the encounters in the appropriate threads on the official PTR forums. Don't worry about bringing your own raid group -- although it is helpful, it's not required. If you need one, just hang around the raid entrance in the Vale of Eternal Blossoms, as it's likely there will be more than a few raid groups looking for people to fill spots. And if you're worried about gear, don't be -- all gear will be scaled to the appropriate ilvl for the encounter.
Read on for more details, including the full schedule of times for individual raid bosses.