Receptionists have gained new recognition as companies' front line in protecting employees from unsavory characters who might try to enter a workplace. So companies across the country are enrolling them in a new kind of training program — one in which receptionists learn about spotting potential terrorists and others out to do harm. A one-day Security Essentials for Front Desk Professionals course has seen a notable rise in the past year and a half, according to National Seminars Group (www.natsem.com), which runs the program.