whipper-in

noun2

1. •
   A huntsman who keeps the hounds from wandering, and whips them in, if necessary, to the chase.

   • “[…] I shall take to hunting a pack of hounds myself after this.” “Do, my dear, and I'll be your whipper-in. I wonder whether Mrs. Proudie would join us.” — 1860
2. •
   One who enforces the discipline of a party, and urges the attendance and support of the members on all necessary occasions.