To JOIN a party, once the owner of the party has sent you an invitation, simply click accept, or deny. If you do not know the friend's nickname, check the chat for it, displayed such as: ' Name Has Connected'and click on invite. Once you're in-game, you and your friends will be on different ships and or planets. Locally, you should still be able to connect to your server using the IP Once the remote tool reports that the port is open, then you may give your IP out to your friends to play. You will also likely need administrative access to any firewalls between the server and the internet In a typical home, this would just be the firewall built into your OS. In some cases the modem itself has a built in router that is often overlooked. You will need administrative access to all routers between the server and the internet In a typical home, you may have up to 2 or more. If the computer you wish to run the server on has a private IP. This allows players to connect to your machine to play Starbound, even if the game is closed provided the server software is still running.

After selecting to open Starbound in Steam the option comes up to launch a dedicated server.

You must be logged into the game with a character for this option to be available. Alternatively you can select 'Invite to game' which will invite them to join your game with whatever character they're playing with. There you'll have everything on your ship with you, but anything on planets will appear as it does in their game, not yours. This will connect you to their in-progress game of Starbound and you'll appear in their universe.

Simply right click a player's name in your friends list and select 'Join Game'. The easiest way to connect to friends playing Starbound is through your Steam friend list.

How to launch a starbound server