Before closing a multiplayer session you should save. The next time then the host can resume the multiplayer game and you or your friend (whoever is hosting the game) can send a steam invite and join from there.
Otherwise you can use the "room" list in the game multiplayer section of the game and find your friend's room there and setup a custom name and password to join. To recognize it all you have to do is decide a custom name and password that both of you can recognize in the list so you can find it and join.
If you use said method, also check the "visibility" in the game options to be sure your room is actually visible in the list and set who can join (public, only friends etc).
Last edited by 3lackrose; 10/10/17 01:46 PM.