The contract was signed after a competitive pitch.
The NAA was set up in April as a subsidiary of the Qualifications and Curriculum Authority to modernise the examination system.
Grant Riches will develop the overall comms strategy, a recruitment and retention campaign and promote the launch of an institute to provide professional training, recognition and development for people involved in assessment.
Grant Riches director Carol Grant said: 'We are setting up an institute to professionalise the role and our recruitment campaign will be run mainly through a website. We will also work on the content and the structure of the site.' Grant also hopes to put together a network of advocates such as head teachers and school governors to back the body.
As well as operating through the website, the campaign will feature press ads, direct mail and events and will be co-designed with the three main examining bodies, Edexcel, AQA and OCR which, between them, write all GCE, GCSE, A-level and national curriculum tests.
The Team, whose client partner Peter Mills leads the account, will concentrate on design, creative and branding work. The MORI Social Research Institute is carrying out research on factors affecting the recruitment and retention of examiners and will monitor the success of the campaign.
NAA managing director Dr Jonathan Ford said: 'High-quality assessment and the work done by examiners is the bedrock of the examination system.
This campaign will enable us to recruit and retain dedicated, professional individuals who will establish a new standard for examiners overall.'