The acquisition also marks the first major development at Grey since David Patton, the agency's newly installed chief executive, joined earlier this summer.
Word-of-mouth marketing, the practice of inviting consumers to experience products and discuss them freely among friends, is one of the fastest-growing communications sectors.
In the US last year, spending on word-of-mouth marketing increased by 21.4% to $4.82bn and the UK is expected to experience similar growth in the future.
The phenomenon has been fuelled by the increased popularity of social networking sites, blogs and instant messaging, which have created channels for people to informally spread messages.
Patton said: "Media proliferation and technological advances have a fundamental influence on our industry. There has been a move away from the traditional selling proposition to big brand ideas executed through multi-channel environments.
"Word-of-mouth marketing is incredibly powerful and greatly enhances the effectiveness of advertising campaigns by adding credibility and authenticity."
Grey and Wildfire have previously worked together on the agency's Procter & Gamble, GSK and Nokia accounts.
The new partnership will see Wildfire move into Grey London's offices, but the agency will retain its name and independence.
Wildfire is headed by Ivan Palmer and Brian Birkhead, and uses data specialist agency Experian to track of word-of-mouth behaviour.
Palmer said: "We are very proud to be a part of Grey London. Word-of-mouth marketing is a pioneering new medium and making it happen requires vision, creative energy and a willingness to explore what's new in order to achieve what's best."