Sean Astin, C. Thomas Howell and Lauren Holly are to guest star on the U.S. science-fiction series \"Alphas,\" Syfy said Thursday. Season 2 is to premiere July 23. Also booked for guest spots are Steve Byers, Kandyse McClure and Noah Reid. \"Alphas\" stars David Strathairn, Malik Yoba, Warren Christie, Azita Ghanizada, Ryan Cartwright and Laura Mennell. The show follows a clandestine group of ordinary people with superhuman mental and physical abilities, a synopsis said.