Simplify the following text for a Grade 9 native English-speaking person.
Original: Brownmiller also participated in civil rights activism , joining CORE and SNCC during the sit-in movement and volunteering for Freedom Summer in 1964 , where she worked on voter registration in Meridian , Mississippi .
Simplified: Brownmiller also participated in civil rights activism . She joined CORE during the sit-in movement and volunteered for Freedom Summer in 1964 .