Do I have to first join a match before I can invite my friends? Is there a way to invite friends to a private match and then switching it to a public match somehow? Help please

Join a game and then invite. Stupid, I know. But without a real lobby system that's teh best you can hope for if you want to join a public game.

Here's what I'd do: Set up your search by choosing a Map (and don't pick Container City because it is very popular) this will create an empty room most times automatically. Then invite your friends. There should be plenty of room!