I don't recall seeing împreună anywhere until now. Is anyone able to explain its relation to cu and why it's apparently necessary for the correct translation?
It is the equivalent of together in the following scenario: you can say with or together with, and similarly you can say cu or împreună cu.
That makes perfect sense. Thanks for the help!
Can you say ..."cu lor" instead of "cu ei"?
No, I don't think so. "Lor" means "their", so also in English it's not right. (I'm Dutch speaker, so correct me if I'm wrong...)