Complex sentence: The first meeting established a Technical Subcommittee chaired by Major General Wilhelm D. Styer, that consisted of Richard C. Tolman, who was Groves's scientific advisor, Chadwick and Jack Mackenzie, the president of the Canadian National Research Council.
Split into simpler sentences: The first meeting established a Technical Subcommittee chaired by Major General Wilhelm D. Styer. The other members were Richard C. Tolman, who was Groves's scientific advisor, and C. J. Mackenzie, the president of the Canadian National Research Council.