In a South Dakota "per stirpes" distribution, a group represents a deceased ancestor. The group takes the proportional share to which the deceased ancestor would have been entitled if still living.
For instance, presume your father named you and your brother as his primary beneficiaries. Your father left you 70% of his assets while your brother's share is 30%. If you brother dies before your father, your brother's share would go to his heirs (likely his children) upon your father's death.