I went to one, was working as part of the entertainment, so let me tell you what it is. The person doing the invite usually is marketing their business online. Basically they are either working for part of the door, since they bring in a lot of business from pof, meetup.com, etc. They might also be the "talent agent" in charge of the cover band, so they sell that to the bar owner as part of the package. They are affiliated with the local bars and promote the events.
With that being said, not all of the people inside the bar will be from PoF or Meet Up, since they are local bars that are pretty popular, but a good 80% will. Look at the people who RSVP, if the ratio is a good 50/50 of women and men, it usually turns out that way at the event, as a lot of women take their friend. Met several who were taken but took their single friend to get her to meet somone new. Avg age was late 30's to mid 40's, though.