You will need to be screened by the host, just like you were about to see an escort.  The attire for the men is usually business casual, if you want to impress the ladies. The ladies are dressed very nicely as well.
The venue is usually a casual sophisticated setting, sometimes closed to the public, and rented out by the host. There usually a cover charge for the gents to offset expenses of renting the venue. There is normally a cash bar, and I would advise you offer to pay for the drinks, if you're talking with a lady.  You might want to look at the Meet and Greet board, for announcements of M&G's in your area. Also you can do a search the M&G board, there have been discussions of this in the past.  

being photoshopped.  You might want to bring some cash along in case you are able to set something up for after the M & G.  Nothing like flirting with many hot women and then spending some private time with a hottie afterwards.