So after spending hours trying to figure out why I was having issues with having someone join lobby through steam and trawling through lots of comments, I finally got it working. I want to put it here because the lecturer completely misses sharing the game with a friend, which is a pretty big error on his part because it causes the host lobby to break when running the game through steam. Its also incredibly annoying since it isn’t a coding problem, so checking your code against the lecturers code is completely useless.
To share the game with a friend you need to go to your game directory in Unity, make a copy or zip of the build folder and send it to them. But before you do that, there is a small text file in the main game directory called steam_appid. Copy that into the build folder first, before you add the game to the steam library or send it to your friends. Your build folder should look something like this:
Adding the steam_appid is important. Without it, when you click host lobby it will come up blank, and you will not be able to invite friends to play.