mob up

verb3

1. •
   (informal) To form a mob (i.e. a large group).
2. •
   (informal) To join, ally with or be beholden to the Mafia.
3. •
   (informal, rare) To gang up on.

   • Gooderson tells me you were not unadjacent to mobbing up R.B.-J. and Sargent in the changing-rooms, Healey. — 1991