camp follower

noun3

1. •
   A person who provides services to those engaged in a particular endeavor.

   • The good fellow is the man with the good character and the object of the camp followers is to see that all the coopers and stave men have a good time. — 1909
2. •
   (historical) A civilian who follows an army, typically a family member of a soldier or someone who provides services to the soldiers, e.g. prostitution, sale of liquor, cooking and laundry.

   • But besides these incompetents, there was always a train of camp followers, —women who followed the camp, beggars, and criminals, whose number was often greater than the number of fighting soldiers. — 1915
3. •
   One who joins a group, movement, or project without being a legitimate member; one who jumps on a bandwagon.

   • I wouldn't discourage my camp followers for worlds! But I watch them pretty carefully, and when they show signs of making money, then I begin to put the screws on. — 1910