Cost sharing between group companies in same state or inter state

Hi... If one company is incurring some expenses and part of the same are allocated between group companies and recovered such cost from group companies, then is it cover under GST as pure agent concept or not. And if GST liability arises then is it required to issue separate invoice by such company to group companies? Expenses such as Rent, Stationery, Rent a cab, Electricity, Telephone charges etc.