Marcia is Cherry's friend and Randy's girlfriend. She's with Cherry at the movies and Pony thinks she's shallow and unprincipled, since she accepts a soda from Dallas, even after he's rude to her and Cherry. Like Cherry, Marcia seems to enjoy hanging out with the gang, and even gives Two-Bit her number (he thinks it's fake and tears it up).
And that's about the extent of Marcia's character. Since Pony doesn't interact with her after the movies that night, we don't see her go through any changes, and her actions don't drive the action or influence the final outcome of the story.