It's obviously more efficient to use Infernal Machines together because one set of keys can be used by multiple people at once. But you want to have Monster Power turned on to improve drop rates for the Organs, and Monster Power cannot be enabled with public games. So how do I create a non-public game with my friends? Do I need to know the user's username or their number too?
Yes, you have to know each others battletag and add them to your friends list. Then its a simple matter of joining your friends game or a inviting them to yours.