These include Lavender, New Zealand's largest independent direct agency, and Family Relations in Finland, which has clients including Volkswagen and L'Oreal.
The other four agencies are M&K Wien in Austria, Gekko in Denmark, Linea Direct Marketing in Hungary and MH Media Solutions in Romania.
Ulrich Bruggeman, chairman of the InterDirect Network and of German agency Bruggemann & Freunde, said: "Since the network was established in 1988 we have seen our numbers consistently increase, yet this is the largest number of new members we have embraced at one time.
"Our combined goal is to be the biggest player in the worldwide direct marketing market and our influence and reach is getting stronger by the day."
Led by managing director Michael Killeen, creative directors Ger Noland and Des Columb, and chairman David Bunworth, Dialogue recently won the inaugural Agency of the Year competition and three golds at the Irish DM Awards in May.
Based in Dublin, the agency has 36 staff and its clients include Bank of Ireland, Lexus, Microsoft and VisitBritain.
Tullo Marshall Warren is the UK member of InterDirect, which admits one member agency per country.
Members must be able to develop international DM campaigns, have a blue-chip client base and operate a broad range of services, including strategy, planning and database consultancy.
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