The agencies include five digital agencies, four search agencies and two full-service agencies as well as a healthcare agency, an integrated agency, a marketing communications agency and an advertising agency.
Eleven of the agencies are based in London, while the other four are based in Edinburgh, Guernsey, Manchester and Bournemouth.
The full list of new members is: Agenda 21; DBD Media; Digitas; Dunnhumby; Dynamic Advertising Group; Glass Partnership; GSW Junction 11; I Spy Search; Pravda Advertising; Reading Room; Sapient Interactive; Steak Media; The Partnership; The Search Works; and The Walker Agency.
Last month, the IPA issued new guidelines that aim to encourage agencies to take control of client relationships by managing the assessment process.The report encourages agencies to let clients know that the way they are treated affects the way they work.
The industry body has advised that both parties should assess each other's performance in a bid to nurture longer-term relationships.
The IPA is the industry body for UK advertising, media and marketing communications agencies.
Its primary role is to negotiate on behalf of its members with media bodies, government departments and unions.