A pretty simple(Now that I see what I ended up writing for this guide, maybe it's not so simple after all, lol)thing to do, but can feel a bit fiddly if you are not familiar with how Xbox Live works on PC.
When you start the game, you'll see a join game option, you want to ignore that, and go into the options and make sure the Multiplayer thru Xbox Live box is checked. Then you want to start your solo game, once it's loaded up, open the Gamebar by pushing both the Windows Key, and G together. Once there, find the person you want to invite into your game by right clicking their name and inviting them under the Game menu.
Once they get into your game, you'll then want to look in the upper left of your screen, you'll see a box under your health bar, where it says your character's name. Click that box and invite the person to your ingame party with their Starbound character's name. When you are joined, either you or they click on the other person's name, and choose to beam to them. Once together, you want to both land on any planet, as you cannot PVP on ships. Once you are both together on the planet, you need to no longer be in an ingame party together, as partied up players cannot fight each other. So whomever the party leader is, clicks on them and removes them. Then you both write /pvp into the game's chatbox to enable pvp, and kill each other.