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What API used to add usergroup under Organisation's unit and its sub-unit?
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: