1. If I invite you to a party, you can bring significant others or friends, as long as you think they'll get along with me.
2. I'd prefer to know whether or not you're coming (and bringing anyone else, and if so who) to an event in advance, because if there are a lot of people who haven't said one way or the other, I will (a) be insecure about it and (b) wind up with enormous quantities of leftover food.
3. If I invite you to anything smaller or more specific than a party (dinner out, gaming of some sort), you can ask if you can bring someone who's into whatever-it-is, but please don't just show up with them.
4. I like taking turns. If I'm doing something one-on-one or in very small groups, I prefer that invitations be reasonably equally distributed; if I've been doing all the inviting (especially if the invitations mostly aren't accepted), I'll eventually stop until the ratio's back to something I like better. This also applies to e-mail, and theoretically to phone calls (although I'm not real fond of telephones). It doesn't apply to parties; I don't mind if you come to my parties but don't ever host anything.
5. If I invite you to do something one-on-one, and you can't make it or don't want to go, it's your turn. If you don't suggest some other time, or other activity, I'll assume you don't want to spend time with me and will probably stop asking you to.
6. If e-mail has a question mark in it, it should get a response. Otherwise, it'll get a response if I have something substantial to say, but I don't always have a lot to say.
7. I like small-group interactions (4-6 people) best, because then I don't have to put out a lot of effort to keep a conversation going and I can still hear everybody.