First of all, there's a Meet & Greet Board here, so go on it and browse.  You'll see where they are and what's involved.  But generally, M&Gs (also known as Mixers) are get-togethers of providers and clients in a casual social setting that usually includes drinks and light food.  They are arranged by well-known hosts and hostess who are almost always local providers and clients.  Screening of all clients is required, so you'll need to have references that check out.  This is to avoid LE getting in.
What usually goes on is conversation, flirting and possibly arranging dates.  It's mainly to let clients have a look at providers so they know more about their appearance in the flesh.  Sometimes there's sex involved and sometimes not.  At one, I was chatting with a provider and another guy and she took him into the bathroom for a free blowjob.  At another, a provider tried to get me to stay by pulling out my cock and sucking it in front of several people.
Then there are M&Gs that take place in strip clubs taken over for the occasion.  At these, some of the girls dance and others will take you into the VIP room for a BJ (not free!) or FS.  There are also pool parties during the summer.
But read the M&G page and you'll see the range.  You'll also read about people's comments on the party the following day.

-- Modified on 6/2/2012 2:24:30 PM