Complex sentence: This idea was presented by a group of white American citizens, called "Friends of the Indian" who lobbied for the assimilation of indigenous people into American society, which they specifically hoped to do by elevating indigenous people to the status of US citizens.
Split into simpler sentences: This idea was presented by a group of white American citizens, called "Friends of the Indian", who lobbied for the assimilation of indigenous people into American society. They specifically hoped to do that by elevating indigenous people to the status of US citizens.