Info:
You can create a 4 man custom party without any mods.
With PartyLimitBegone Multiplayer Mod you can create up to 8 custom characters in the same party (you can find this mod at Nexus).
If you try to create a custom party with more than 4 characters some characters might spawn outside the map. But after you discover your first Waypoint you can easily teleport your "out of the map characters" there.
I have no idea how to make this on console.
Tips:
If your computer lags alot while doing this then plan your characters race, class, gender, etc. before starting this tutorial. You can also edit your characters appearences later on at the camp mirror.
Thanks for watching!