Final answer:
Dr. Martin Luther King Jr. was the leader who spoke about the role of peaceful and grassroots efforts to include African Americans in society, notably through the Montgomery Bus Boycott initiated by Rosa Parks.
Step-by-step explanation:
The individual who spoke about how the role of peaceful (P) and grassroots (G) activities was to demonstrate that African Americans were an integral part of society was Dr. Martin Luther King Jr.. During the Civil Rights Movement, he advocated for nonviolent protest and promoted community activism to fight against racial segregation and injustice.
A significant example of this nonviolent resistance was the Montgomery Bus Boycott, which was sparked by Rosa Parks' arrest for refusing to give up her seat to a white person. The boycott lasted for 381 days and was successful due to the unity and determination of the African American community and the leadership of individuals like Rosa Parks, E.D. Nixon, Jo Ann Robinson, and Dr. King himself. This protest led to a Supreme Court decision that racial segregation on public transportation was unconstitutional.