I never needed to arrange for someone from my office to show up, would just let the agent holding it open know my clients were coming by (unless there were a multitude of houses my clients wanted to see outside of this house and if so I'd have made arrangements prior to my not being available). I never expected a buyer client of mine to start drilling the open house listing agent on site, most times the agent holding it open isn't even the sellers agent, just an office mate trying to pick up unrepresented buyers, so they'd likely have no true knowledge of the property beyond what the MLS sheet spits out