Complex sentence: The national organizer of the group was Lovett Fort-Whiteman, who had previously attended the 6th World Congress of the Communist International as a delegate of the Communist Party, USA in 1928 before staying on to attend the Comintern's International Lenin School in Moscow.
Split into simpler sentences: The national organizer of the group was Lovett Fort-Whiteman, who had previously attended the 6th World Congress of the Communist International as a delegate of the Communist Party, USA in 1928. Following completion of the congress, Fort-Whiteman stayed on staying on to attend the Comintern's International Lenin School in Moscow.