We were really lucky I guess. However, we rented out the firehouse banquet hall and had parties for probably 125-150 people. The last few years we did it we set themes. The last few were "come as your favorite celebrity, author, singer, etc" so it was a lot less restrictive. I had movie themed food, it was BYOB, and I made Academy Award statue centerpieces. I made a walk of fame for the entryway, and had either a game or just awarded prizes. The game was that everyone received a number when they arrived to pin on their clothes. Everyone had to guess who each person was and whoever had the most correct guesses won the prize. The prize was a gift basket of a Horror movie, 2 of the Halloween goblets from Dollar Tree, a bottle of wine, some popcorn and movie snacks, etc. I never asked for an RSVP. However, if you want people to respond, just let them know that if they don't reply, they are not allowed in. Have someone at the door, like at the fancy nightclubs, checking off people's names when they arrive.