McNamara (pictured, left), who will appear in the upcoming “Maze Runner” follow-up “The Scorch Trials,” joins the ABC Family drama as Kat, an attractive, ruthlessly ambitious piano prodigy Brandon (David Lambert) meets at Idyllwild. McNamara is a familiar face to the Disney/ABC family, previously guest starring on on Disney XD’s comedy “Kickin’ It,” plus Disney Channel’s “Jessie” and “Girl vs. Monster” telepic.
Johnson (pictured, right), currently on ABC’s “Nashville,” has been cast as Tom, an overzealous but very talented musical composer who Brandon befriends at Idyllwild.
Williamson (“The Goldbergs,” “Switched at Birth”) will recur as AJ, a runaway who Callie (Maia Mitchell) bonds with at the drop-in center.
“The Fosters” was greenlit for Season 3 this past January.