Groves is taking some of 64k's most important clients with him to McCann.
These include Nescafe Original, Yoplait Dairy Crest's Frubes brand and the pharmaceutical giant Bayer.
Groves has been brought on board to bring a new-media consultancy and creative dimension and boost the skills that McCann can offer its clients. Nescafe is a key McCann client.
"McCann Online will add a dimension to the London agency and allow it to look to online advertising directly as an internal resource," Groves said.
Groves, who will maintain a minority stake in 64k, said the company would continue to function as a production house but it would not pitch for new clients in its own right.
McCann does have its own digital creative division, called McCann-I, which has clients including Durex. However, it operates independently from London and pitches for its own work as part of McCann-Erickson Manchester.
In April, 64k launched the new-media campaign to support McCann's television ad campaign featuring Afro-man Charles and the "Hairy Old Cortina". It also handled the digital creative work for Nescafe Original's £10 million ad campaign to promote the new-look jar design and relaunch of its brand.