Smith will be part of the Bell Pottinger Wired team and work with Bell Pottinger’s financial and corporate teams, while Armstrong will be part of the Bell Pottinger Political division.
Smith led his own digital marketing agency, Initio Consulting, before selling it to Haggie Partners, which he joined as head of digital.
Armstrong was a special adviser to Michael Gove, the former secretary of state for education, from December 2013 until Gove was moved to become chief whip.
Before this she was head of the political section of the Conservative Research Department and worked on Boris Johnson's successful 2012 bid for re-election as London Mayor.
James Henderson, Bell Pottinger Group CEO, said: "Harvey and Beth will help us build further on our approach of delivering truly integrated and cutting-edge communications, providing clients with an expert guide through the complex markets in which they operate."