Sounds awful, and I know what it's like to be excluded.
But are you absolutely sure they all have your number saved in their phone? And if so, did they all have your number saved at the same time, or were you added a short time later? If that's the case, whoever set it up might not have realised you were on the app and it's just an oversight. A similar thing happened to me, where no one actually realised I wasn't part of the whatsapp group because I didn't have whatsapp at the time the group was created.
I have to agree with those who say it's probably one of those things that have just grown, and it's coincidence you haven't yet been included. Maybe people were included when the group wanted to ask them a work related question, and that's how it grew. You sound like you're on the ball with work so maybe they have never had to do that with you. It might have started with people on the same team or working on the same project, and then more people were added until it became this.
I know it's not nice, but a group of eight is a big group, and it might not be noticeable to everyone who isn't there. Maybe they think you're in the group and just don't speak in it.
Clutching at straws, I know, and maybe it's wrong that I'm trying to give them the benefit of the doubt. But from what you've said it sounds like you all get on well, and if there isn't a solid reason as to why you've been left out, maybe it's fairly likely it's just a mistake. Whatever the issue, I feel for you, being excluded is the worst.