Pearce will play LGBT activist Cleve Jones, Parker will play women's rights leader Roma Guy and Griffiths will play her wife, social-justice activist Diane.
The roles of African American community organizer Ken Jones and transgender activist Cecelia Chung are still to be cast, ABC said in a press release.
"When We Rise chronicles the real-life personal and political struggles, set-backs and triumphs of a diverse family of LGBT men and women who helped pioneer one of the last legs of the U.S. Civil Rights movement from its turbulent infancy in the 20th century to the once unfathomable successes of today," a synopsis said.
Cleve Jones was a young Quaker from Arizona, who joined the gay-liberation movement in 1972 and was befriended by pioneer gay-rights leader Harvey Milk. He also founded the Names Project AIDS Memorial Quilt and led the National March for Equality in 2009.
Black and Van Sant previously collaborated on the 2008 Harvey Milk biopic Milk.