afaik it's random

You can disable this in the 1.1 galaxy forge. But I haven't figured out how to be anything except player 0 in single player.

In galaxy forge 1.1, from the defualt menu (where you can add description and stuff), there is a heading that is something like random position, which by default is true. Turn it to false, and the players will always get assigned to the same planet. (The first player becomes player 0 in the forge, second player becomes player 1 etc.) Unfortunately, I can't figure out how to get out of the first slot in single player, meaning you always get player 0. Probably can do something in the forge player menu but I haven't had the energy to figure it out yet.

Let me point out that if the game had custom map auto-download, this wouldn't be an issue since there would be many custom maps to choose from where everyone either gets a similar or good starting position and/or people start near their teammates, etc.