The YPM, as it is often abbreviated by fans, was thought to have begun with Aftran 942, the Yeerk Cassie persuaded to leave her host, a girl named Karen, and live without a host. Later, Yeerks with hosts became a part of the movement, as evidenced in The Sickness, the book in which the movement's existence is first revealed. Yeerks involved in the Peace Movement are considered traitors by their fellow Yeerks and are subject to execution if they are discovered.
It isn't known what happened to the YPM members after the war, the final mention is that the Animorphs lost contact with Mr. Tidwell and the other YPM members during the final phase of the war.
The group's appearance in the TV series is similar to its appearance in the books, although it is first discovered when Mr. Tidwell attempts to aid Tom in escaping the Yeerks, rather than the events of The Sickness.