If there are M&Gs on the calendar, how could I be invited?

TER members do though. There is in fact a M&G on TER that you may have seen.  

You generally need to be verified by the host in some manner to be invited. You can check the M&G board to see if there is one planned near you and follow the instructions the host lays out to try to be included. Some are publicized and others are very UTR and you need to be personally invited to attend.  

...every few months.  Check the Meet & Greet board.  Here's a link to one of Jimmy's posts.  Email him and get on the invitation list...

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TER does not M&G specifically, but most of the major cities have an event.  You need to latch on to a well connected hobbyist or lady in your city.  Occasionally there will be something posted on the boards but this is a recipe for disaster when people start talking about M&G before they happen.   Just do a little homework and I am sure you will discover one.