You may designate up to eight individuals of your choice as Companions, granting 3 random 0-CP perks, a random 100-CP perk, and 60% of your total CP to spend as they see fit. They may choose to either draw from the same crafting-related perks and items that you do, or specialize into a different category, such as combat perks, magic perks, and so on.
"Property"-type items, such as buildings, landmasses and the Personal Reality, are shared by everyone, meaning that only one person in the Forge has to purchase them for everybody to benefit. If you have also purchased Blood and Sweat or Increased CP/Frequency, they will also benefit from those as well.