CanYou - she's not in a group of 8. She's part of a whole class - even if it's a ridiculously small class, at a private school or something, it's still going to be twice the number who are invited.
Of course I've been in situations where people are talking about an event they are going to or have been to. No, I don't feel it's rude, I just think 'I hope they have (or had) a nice time' - I have lots of different people that I know from lots of different things I do, and just because I'm friendly with people at one thing, I wouldn't expect them all to invite me to their parties. I'm perfectly capable of being friendly without being needy, and it seems my dc are too.
As I've said over and over - like Precious just above here - I've had 3 dc go through Primary school, all very different, and, no, not all invited to lots of parties, but I've just never been aware of who was having parties if my dc didn't get an invitation. If ever any of them mentioned that it was x's party on Saturday or something and they were a bit sad they weren't invited, I'd have said something along the lines of "Oh well, everyone doesn't have the same sorts of parties, numbers are often limited and they might have lots of cousins or something they need to invite" and then distracted them with something else.
I agree with Claraschu here : I do think that it is important to play this sort of thing down for your child, and make 100% sure you are not in any way increasing it's importance in her perceptions. You must use your wiles to trick her into forgetting or not noticing such things in the future. Sometimes children are not even as bothered as adults are about parties . It's up to you if your dc sees this as a big thing or not.