Distributorship agreement between Claimants and a company not party to the arbitration / Defendant not a signatory to the contract / Extension of the arbitral clause to Defendant (yes) / Presumption of assignment of the contract by the signatory company to Defendant inferred from the parties' behaviour (yes) / Performance of the contract by Defendant is evidence of identity between Defendant and the company signatory to the contract / Identical share ownership and representation of both companies are indicative of the existence of a group of companies / Application of international trade usages / Common will of the parties