Depending upon the locality, many meeting / party places have a placard from the local fire marshal, stating the maximum number of people for the facility. It is there for this very reason as well as for evacuation purposes. Human nature being what it is, people will ignore this unless there is someone in charge and willing to get tough.
Not just for evacuation purposes. Places of public assembly designed, IIRC, for population loads greater than 50 people also have more stringent structural loading minimal standards which are to be designed with greater lbs/sf loads.
Thank you for properly spelling fire marshal. That may be a first for FR.