The deal with US group Horizon Media will add £700m in billings to CMG, taking the total for the expanded network to £2.1bn globally.
Horizon has its headquarters in New York with offices in Orlando, Florida, Atlanta and Amsterdam.
Last month, Spanish independent Cencomed, with billings of more than £48m and clients including UIP and Dreamworks, joined Eurolab.
In Russia, the network added Sovero Media, with billings of £17m Pepsi Bottling Group, SAB Miller and Pernod Ricard as clients.
The group also expanded into Canada with Cossette Media, which bills £250m for clients including Coca-Cola and Nike Canada.
In Latvia, Lithuania and Estonia the network partnered with CMS (Creative Media Services) adding £13m of billings and adding clients such as Adidas, Hyundai and L'Oreal.