Larry A. Kohn is quitting as partner at headhunter Heidrick & Struggles to join Quellos Group LLC, said David Barrett, partner-in-charge at Heidrick & Struggles. He starts May 1. Sources said Quellos is looking to increase its institutional business. In March, Quellos hired Bruce M. Dresner, former chief investment officer at Columbia University, New York. Mr. Kohn will market Quellos to pension funds, foundations and endowments, sources said.