I'm sorry your dd is being bullied. It's really horrible for her. The first two years of secondary school is a classic time for this to happen while everyone sorts out their friendship groups.
You need to build up your dd's confidence and support her but don't get involved with the minutiae, but if you decide you do need to step in, do it once only and make sure it is effective. Don't keep trying to control everything from afar because it is impossible to do and makes the situation worse.
My dd's friendship group started making on-line nasty comments at the same age which got out of hand on one occasion and just as we were about to post a message, one of the other mother's stepped in with a timely message on WhatsApp to say that "all of us parents are checking in on this, so please be kind to each other, nasty comments are not acceptable" which seemed to do the trick.
I called the other mother to thank her and we agreed to mention the bullying and ill-feeling to the school together the next day. The girls' teacher then set aside an hour one afternoon to focus on kindness and bullying and gave a very effective talk about being kind to one anorher, how horrible it is for lots of DC to hang up on one DC etc etc, they all apologised, and the problem was eradicated.
The point is that my daughter was the subject of some (not all) of the nasty comments and the daughter of the other mother who stepped in had made some (not all) nasty comments to my DD to which my DD had retaliated in kind. And pretty much all of the DC had made nasty comments to one another at some pount. However, me and the other mother didn't take sides exclusively defending our own children, we recognised this was all the children having a go at each other, and it was up to us as parents to set them straight all together by putting on a united front to set a good example. If we had taken sides, it would have exacerbated the problem.
Obviously it's very different if all the DC ostracize and continue to bully one child and it's hard to tell from your description what is going on with your dd and her friends precisely, but it could be that this is a group of eleven year olds being eleven year olds who need to be set straight by all the parents together, and their teacher.