This is a short thing for people who never used it like I did.

This game was made for local co-op, so when you invite someone to play with you on your friends list you send them a link. With this link they will join a sort of 'lobby' where you can add up to four or so players, you should only add one other player as the game was only meant for two player co-op.

In this little screen you see your two icons next to a mouse and keyboard icon, these two icons are lit up for the host but not the person who joined, click the two icons for the new friend to allow them to control in the game.

If you press F2 you realize both of you control the same character, this is because the two characters have different control settings on your keyboard. So if you're not using a controller, you need to use different keys for your character. The host needs to bind these keys accordingly

After all of that you will be able to play with your friend as if you were sharing a keyboard.

This isn't much or too well written, but I had to take time to figure this out and it was frustrating. Hope this helps someone, take care now <3