PS4 Lobby/ Resuming Play Online

Has anyone found it difficult to figure out how to get friends into your lobby after it has started? I am assuming now after fooling around with it that you have to start the game with the friends that you want in the game so it can populate the environment for them as well. I have not found a way for friends to join after a game has been started. Not a huge deal, but just something I noticed.

The big deal is, I cannot for the life of me figure out how to get friends back into a game that we were previously playing in. Once we leave the lobby and it is saved, I go back to continue previous map, and it starts it but it doesnt give me the option to start it with friends. I really think that they need to add "save slots" so we can choose what save we want to load. It doesnt matter a lot, because of how narrow the focus of the game is, but I am not sure if the objectives start over every time you start the map. If they do not, then it is no big deal.

Anyone know how to resume a multiplayer session and get your friends into it as well? Do they just click on resume last session? It sucks if that is the case, because when you play with different people, they wont have the same level of progress as you.

Hi ! its been a while since you posted this so maybe you found a fix or a way to make it work ? Im having the same problem :S