Most recently I was invited at the SF Pro Store when I spent a whole bunch, the manager invited me and put me on the list. I didn't even know it was coming up. I was invited to Sundressing by calling and asking to be put on the list, as soon as I heard about it on Specktra. I've asked the MA's when I'm at the store shopping and been put on the list. I've also received invites the in the mail (but not very often), which I believe is done randomly from the database, as posted above.
The parties are only held at PRO Stores and at freestanding stores. The counters hold events, but they are not preview parties. Instead, they are just opportunities to get your makeup done with the new collection, as soon as the collection launches.
As far as which stores have parties, I think it depends on the collection and your area. For example, the Sunvalley Mall store in Concord (San Francisco Bay Area) was the only store in the area to have a preview party for Sundressing, even though we have a PRO Store and 3-4 freestanding stores in the Bay Area. I'm wondering how MAC decides which stores get to host the party. I know, for Untamed, there will be a party at the SF Pro Store and the Berkeley freestanding store.
Hope that helps, since evidently my link did not.