Hello. I'm new to this forum and I'm very big fan of Jazz Jackrabbit.
It was my first game on PC and I played both games multiple times over the years.

This time I wanted to play with my friend JJ1 with OpenJazz multiplayer coop but I don't know how.
I was googling everything but I never found real answer.

I don't have multiplayer option when I launch OpenJazz, can someone please help me? Thanks in advance.

Welcome! It's pretty straightforward: from the OpenJazz main menu, pick "NEW GAME," then pick "NEW CO-OP GAME" if you want to be the one who hosts the server, or "JOIN GAME" if you want your friend to host the server and you want to join it.

One good guy uploaded his files and I downloaded them and it's working. Problem was that new github version of OpenJazz does not include multiplayer option when starting a new game :/

If you create an issue with the menu problem you're experiencing they can take a look at it.