Invitation theory is still a discipline in its infancy, and geek culture really pushes the envelope.
Posting publicly about a trip to a restaurant results in inviting undesirables (we really hate "Johnny" because he always "forgets" to pay and drones on incessantly about Linux), or incompatibles (Billy and Suzie hate each other and won't voluntarily share the same space, so inviting both results in neither coming). Private communications invariably leave a few folks off the list, and Johnny shows up anyway for the free meal. (Johnny has, somehow, traded in all his conversational and common sense skills for an innate ability to know when any group of acquaintances is having lunch, anywhere.) Limited invite lists frequently include undesirables to the exclusion of desirables (because you dearly love Emily, except she's got *totally* lame taste in men and you have to invite Joe to avoid a personal tangle. This, of course, doesn't leave enough chairs at the table for Fred, who you would otherwise invite.)
You send out emails to a bunch of people asking them to a restaurant for dinner, and you just hope you aren't the one who shows up first to ask for the table. Because you never know whether to order a table of four, or ten, or if your table of ten will have four more people try to crowd in after dinner's ordered. RSVP's can give you a base number, but not a particularly reliable one, and past performance is no indication of future behavior. Chinese food adds another layer of complexity, with sharing/hoarding/vegitarianism considerations -- and despite their usual mathematical skills, geeks seem unable to add both tax *and* tip to their order, along with a peculiar bout of amnesia regarding beverages. Alternately, an end price can be divvied up evenly, which is OH SO FAIR when the high paid engineer ordered wine and six appetizers and the grad student got soup and a glass of water in hopes of paying PG&E this month. Of course, the grad student *did* have one of the popcorn shrimp the engineer offered him (A morsel of protein! Oh rapturous joy!), so he's no saint in all this. But I digress.
Then, of course, is the "geek vacation" where everyone goes out of town for skiing or Disneyland or whatever. These require a great deal of planning and each person who says he/she is going had better not just "bail" because rental houses don't come cheap. And as we all get older and fussier, fewer people want to be the one to crash on the floor. Once you've set up an excursion, you face those who feel they should have been invited along in the first place, and those who think they should get to tag along at any time. So people get lined up (or nixed out) early on, and don't want to say anything about it in public lest they inspire jealousy. Then they have such a great time that they tell everyone about all the cool things they did and everyone left off the list feels like a leper with hepatitis-A. Advice: don't be tight lipped about your trip at any stage to save anyone's feelings. We're geeks: it is in our nature to know where you are at all times. Just say "the trip is full", and if someone else wants to arrange a second house for the same weekend you can make arrangements to meet at the same resort/event/whatever (and *they* can stress over who to invite).
Finally, I'd just like to defer to Miss Manners on all of this, and say that those who want to join events organized by others should organize events themselves, and if the offers are not reciprocated later to remove those individuals from future invite lists. And that smaller affairs require cycling through different people over time, so one shouldn't take offense at not being invited to an event or two, until the pattern becomes obvious enough that you can tell they're not that sort of friend.
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