What API used to add usergroup under Organisation's unit and its subunit?

Organizations[<EntityName>].AddMember({
	name: <org/org unit name> /* STRING */,
	member: <user/usergroup> /* STRING */,
	type: <"User" or "Group">/* STRING */
});

FYI: You can lookup all available services of an entity in the "Me/Entities" section when creating a service: