The Terps are left with Josh Jackson and Lance LeGendre as their lone scholarship quarterbacks.
Defensive back Osita Smith, offensive lineman Amelio Moran and defensive lineman Almosse Titi all signed Wednesday.
Pigrome played in 11 games last season, making three starts.
The Maryland football legend ends his 14-year career with two Pro Bowls and a Super Bowl championship.
The Terps added two recruits to their 2021 class and one to their 2020 class.
Castleberry didn't receive a carry for Michigan last season.
Harrison suffered a season-ending knee injury against Penn State on Sept. 27.
Jacobs spent six seasons with the Terps from 2013-2018.
Randy White played for the Terps in the 1970s and was a two-time All-American.