1. Make sure you, and all the players you wish to play with, all have the same mods.
2. Make sure everyone uses steam, otherwise, you're screwed.
3. Make sure everyone is friends on steam.
4. Make sure everyone owns Starbound.

1. Enter the game.
2. Have your friends enter the game.
(HOST)
3. Press Play/Singleplayer
4.Open steam with Shift+Tab
5. Invite all the players you wish to play with.
6. Wait for them to enter.
(ALL)
7. Once all the steps above are completed, you're good to go!
(See next section for party details/help)

If you just want them to be on your server, ignore text below.
1. Click the person + symbol near the top right.
2. Enter their Starbound character's name.
3. Press invite.
4. Repeat steps 1-3 if you have multiple members.