Oh man, I'd be a sad cupcake too, that's really upsetting. There was nothing accidental about their decision to not show up, they're excluding you on purpose, and they probably have a ringleader. The choice about whether or not you get to stay in the group is probably not up to you, so you might as well be perfectly honest with them about how they made you feel if they're going to push you out anyway.
Do you have friends within that group who will give you a heads up about what's going on? Otherwise use the group chat and address them as one. I'd appeal to their Fe and Fi, meaning let them know how their behavior was inappropriate in the general social sense, and also how it affected you personally. Just be honest and completely straightforward, let them know what they did had real repercussions on someone they say is their "friend". Maybe something like, "I need to let you know that cancelling on me last minute really hurt my feelings, especially when I see you all get together without me immediately afterward. After spending all day trying to make everyone's favorite foods and treats so I can show you how much I appreciate you, it really, really sucks when you all agree to ditch me without letting me know what's going on, or that you were even making alternate plans. If X was sick, I would have like to have seen her too and brought her something! I've seen other photos of you all together, even after I've been told you don't have time to meet. How do you think that makes me feel? I'm trying really hard to be a good friend, but I feel like I've been cut out of the group and I don't know why. I'd much rather just be told what I've done wrong than repeatedly get my hopes up only to be excluded." That might be laying it on a bit thick, I don't know. Either you'll get a lot of backpedaling and they'll try to justify their behavior, they'll feel like they're being victimized and turn it around on you so they feel more justified in their behavior, or one (or more) of them will step up and try to be better friends or explain things. The odds aren't great that they'll let you back in, but don't let them shove you out without knowing exactly how their behavior is affecting someone they call a friend. I wish you the best of luck :(