Riverside’s brief will be to gather support and funding for upgrades to the A120 road, which links the port of Harwich in Suffolk with Stansted Airport in Essex.
The brief will cover public affairs and corporate comms as well as working with regional stakeholders and central government.
Dave McCullough, MD of Riverside, said: "The Government has made a lot of noise about infrastructure spending, and nowhere is this needed more than the A120. Our key role will be to assist HGP with telling its story and proving its case over the coming months."
HGP was created in 2001 by stakeholders in north Essex and south Suffolk to promote the area an economic hub.
Steve Clarke, executive director of HGP, said Riverside’s government contacts and successful campaigning record made it the "ideal fit for HGP".