I know someone who has gotten several random guild invites, and I've gotten a "Hi there, want to join my guild?" once. No clue why that happens...my guild invited a couple of people who just came up and said "Can I join your guild?". We learned our lesson; pretty soon the guild changed their policy to "no more inviting people we've never grouped with."
Someone randomly invited me to join their group the other day, and I declined. A minute later, I get a tell saying "why?". Now, that annoyed me slightly...if they don't want to talk to me first, then I don't want to give a reason for not joining.
Oh, and then there was the guy who got mad because my mage wouldn't group with him and cure his plague...