I remember going through something similar--I think I still do! And my 9-year old stepdaughter is going through the same thing.
The only thing I can come up with is people seem to be very self-centered and rarely think of others. I think it's a result of our society nowadays and the lack of manners being taught. I would never think to leave someone out, and I remember always being the one who reached out and remembered to include others when a group of us were going out to do something.
I totally remember sitting at home, waiting for the phone call from a friend who was going to solidify the plans and never getting the call! Then later on you find out everyone did something else without you.
I feel like I'm always doing the asking for playdates for my SD. We hardly ever get invited over! The playdates happen, but only because I make it happen.
The only advice I have is for your daughter to keep reaching out. From my observation, a group dynamic goes something like this:
Every kid in the group feels a little unsure and a little pressure. They are only concerned with themselves and being accepted, not on purpose, just because they are young. They all go out and do something and the experience bonds them. The next day they are all laughing over what happened, etc. Within the group, friendships will form, with some members being closer to other members. If your daughter makes plans she will have a chance to be a part of the group and get to know the members. The experiences will bond them, missing an experience seems to ostracize that member. Your daughter is giving herself the chance to get to know some of the group members better and maybe make a close friend. If after several outings she doesn't feel like they are her type of people then she can seek friends elsewhere. But at least she should give it a try.
Encourage her to make plans and invite her friends. Not only will she become a leader but she could uncover a new friend!
I felt kind of pushy by always making plans with this one mom, but my daughter liked her daughter and they both got along great. After quite a few playdates (arranged by me) my daughter was invited to her birthday party of only a few select girls. Now this girl and my daughter are good friends and the mom recognizes it. I guess it takes some pushing but the results are worth it!