"aufbrechen" is in this case "to leave a place", "to set off", "to hit the road"... For example: "Ich muss heute Nacht noch aufbrechen, damit ich nicht in den Stau komme." (I have to leave this place this night, otherwise I would get into the traffic jam.) I hope I am not completly wrong, but you have to use it with "sein". So: "Dann sind sie gemeinsam aufgebrochen" (Please correct my someone if I'm wrong, otherwise it sounds so strange.) Hope it helped a little bit. :)