GTs were done in different forms. The ones that was the most common was the bar/venue. The place would be reserved for "us", and the bar would make its money by selling alcohol. Of course it needed to be a significant group. Once a party was held in a private house of 3 floors and PERSONALLY that was my favorite because while the majority of the party was on the top floor with loud music and dancing etc, we could go around and just lay casually other places to have more "quiet spaces" to talk. There was even peoples sitting in the stairs. If you saw the parties in movies like American Pie, it was almost like this, wich i tought was super cool. That specific party also had room reserved for 30 minutes quickies with workers that wanted to work that evening (they had some kinda glowy stick around their neck).
Honestly, before actually making preparation for one, i think it would be best to gauge the respond of the SPs in general. Because they are the ones who would make the guys come (no pun intended) in bigger numbers.