For US and especially Las Vegas events: usually the venue has their own professional security staff, some with law enforcement status of some kind, mostly with just security guard accreditation, some armed, some medically trained, etc. Venue also provides other paid employee or contractor staff for some things like cleaning, food service, etc. Especially in Las Vegas, this is highly unionized and regulated (to the point where connecting network and power cables within your own booth at a convention center event is prohibited and must be done by venue electricians at something like $400/hr)
Then hacker conferences like defcon have their own volunteer staff of various kinds. These usually are doing crowd control and information, but occasionally get involved in attendee drunken or stupid incidents, usually with lesser consequences to attendees.
Some high profile attendees (NSA head, John McAfee, etc) have their own personal security; goons/volunteers then worked as a buffer between those people and attendees. (I did this for a McAfee event at BSides which was super fun because his armed security were also high on methamphetamine and erratic)
