Supposedly The Grateful Dead recommended the Hells Angels to Mick Jagger as security. I guess the Hells Angels had been security for The Grateful Dead a number of times before that.
The interesting thing about that is that when the members of the Grateful Dead were getting out of the chopper at Altamont,someone told them what was going on and how the Hells Angels were being so violent,so the Grateful Dead got back on the chopper and left!