Stiffing the tip as groups get larger ... like the prisoner's dilemma but ...?
Bystander Effect

The Bystander Effect could be your answer. The more people, the less personal responsibility. 


Person gets assaulted and mugged on a crowded street in broad daylight, nobody calls the cops. 
Software bugs slow company productivity almost to a halt, many employees, nobody reports the problem. 




Tragedy of the Commons

The tragedy of the commons has been studied from many perspectives. You can use that commons dilemma link as a jumping off point to hopefully happen across something relevant. But I would start by looking at the psychology behind the bystander effect.