I also fall back on availability and interest for stuff like this. If it's an event that a close friend is hosting, I ask if I can bring a plus two, and contribute something else to the event (food, drink, etc) to make up for the extra number. If it's a business-y thing where I can purchase as many tickets as I want, I bring more than a plus one if everyone wants to go. If it's a hard plus one, then typically Elemental comes because other people live out of town. We just talk about it, and make sure nobody feels left out, and do something as a group afterwards/plan another special date if they do.