"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. :)

I'm just a conversation-teacher, so I also have no idea of german grammar. ;) But what I can tell you, don't worry about that so much. If you not talking to a grammarnazi, then everyone will understand you or even better: no one will see it as a mistake. "Herr Schafer ist nicht da. Er arbeitet." I was living in eastgermany (grow up there) and westgermany (lived there since I was 18 years old) and there I notices something. People in the west say "der, die, das" often to people. Like a westgerman could say: "Der arbeitet." and it sounds unpolite for a eastgerman. (my opinion!) But I got so used to it, that I also say something like: "Emma? Achja, die ist schon zu Hause." (Correct me if I'm wrong) but my personal "eastgerman-feeling" would have said, that's pretty unpolite. :D But I think, or I just got used to it, that there is no problem to say both. So what I want to say, german has so complicated grammar, don't think about this grammarpoint too much. Der, die, das, er, sie, es. Mostly it doesn't matter in this case. Germans also don't speak like the grammarrules are, unless you are a teacher or somethingelse. I know I couldn't help so much, but maybe someone with grammarknowledge will answer too. I just want to tell you: in this case it doesn't matter at germanys streets(talking).