Starbound doens’t have DRM which means you can just go into the folder that Starbound is downloaded and run the server or client exe without Steam running.
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The servers are not centralized or run by Chucklefish, meaning all servers will be player moderated public or private.
Each instance of the game is housed on a server, which players must connect to in order to play together. Multiplayer refers to playing together with other player controlled characters in game. Once you’re all in a group, party members can click on the avatars of each other to teleport to that group member’s ship. Enter the name of your friend (s) to invite them to a party. Head to the upper left of your screen, near your player avatar, and click on the plus sign.