camp follower

One who follows a group without being part of it. The practice originated with the families of recruits, prostitutes, and traveling merchants, who would settle near a military encampment. Later it was extended to others who benefited from military installations. The term itself may come from a letter written by the duke of Wellington in 1810.
In mid-twentieth-century America the camp followers of rock musicians and other entertainers, mostly young women who followed their idols on tour, acquired the name groupie, which then was extended to any ardent fan.