In order to have your friends join your family you will need two things:
Have an NPC family member. Players don't spontaneously generate new characters when they join, they have to inhabit an existing NPC. So, if you want a friend to be part of your family, you'll need a family first. This can be a spouse, child, sibling, niece/nephew, etc.
Provide the code. Your immediate family (two generations in either direction) is considered "locked" to other players unless you invite them. Each of your eligible family members has a code. By giving that code to another player you allow them to spark their soul into that family member. Keep in mind that if you give your sister's code away to a player, any of your sister's NPC children (your nieces/nephews) become locked to that new player and you lose access to that entire branch of your family.