First of all, you and your friends need to register a klei account, which is what you need when you enter online games.

After logging in, you can create a world for your friends to join. Click the host on the left side of the homepage, select a world vacancy, adjust all settings, click Create World, and wait a moment.

At this point, you will enter the role selection interface. Switch back to steam, click on the friend at the bottom right, click on the small triangle next to the friend you want to invite, and select "Invite to join the game" in the menu that appears. After your friends accept it, you can start the game.

Note that this game server sucks, except for the host (

Founder), others will get a little stuck when playing.