Grey Healthcare Group (GHG) has acquired Boston-based online advertising and marketing firm Catalyst. Financial terms of the deal were not disclosed.Catalyst’s founders, Heather Frahm and Beth LeTendre, will continue to lead the firm from its Boston office, managing a client base that includes Biogen Idec, Novartis and Pfizer. LeTendre said in a news release announcing the deal, “Joining GHG is a great fit for our company and allows us an unprecedented opportunity to expand our offerings to its growing client list.”The addition of Catalyst also gives GHG a boost in the area of search-engine marketing (SEM), said GHG CEO Lynn O’Connor Vos.“SEM is one of the fastest growing areas in the digital media space…it has developed search capabilities that enable clients to reach the most receptive areas of the market and provide the greatest ROI.”