You will be able to join an existing family By using the child codes of children in the family.
When you select an NPC during Character Creation you may select an NPC open to anyone, or you may select a closed NPC if you have their NPC code. Closed NPCs and the associated NPC Codes is tied to the family system.
Otherwise you can start as a different character geographically close to your friend and using your surname create a family of your own. Then simply marry into your friends family.
The communities that have formed and have extensive backstories may not be able to be formed as expected once the game launches. There are certain constraints such as available family size that may change how some relationships are set up. Instead of being siblings some may have to marry into the fam.
