Some open-ended questions for you to ask her when the subject comes up:
Who does she like to play with? Does she feel good when she plays with them? If not, then ask her who she does feel good playing with. If so, why?
You can notice when her nicer friends are around, and remark on how kindly or considerate they are toward your daughter. "Rachel must really like being friends with you. She gave you a big goodbye wave!" or "It sounds like you had fun playing with Isabella and Christine. You were smiling when you talked about them. They sound like fun friends." When the harder moments come up, when she faces the hurt and rejection, you can give her empathy: "That's hard. I don't like being left out of fun stuff either. (and then ask a question.) What do you think you could do/say if this happens again? Let's write it down." Don't offer any criticism of her ideas, just write them all down and then focus on the two or so that seem most effective from your parental perspective. Then ask her which ones she likes most and would like to try. Talk about what might happen.
Ultimately, one of our jobs as parents is to teach our children the universal truth that we can't control the actions of other, only how we choose to respond to them. While I would never expect a six year old to maturely contemplate in the moment what their response would be instead of rightfully being upset at being bullied, we also want to empower them to feel that they aren't just *stuck*, your daughter has options. She'll have to learn some of this the hard way.
And if it were me, I'd also cue the teacher in and have them talk to the playground monitor. If your daughter is being bullied, it's good for the teacher to know. And if she's following them around, the teacher might have a 'special job' or another distraction for your daughter. Hard to do in a class of 25+ kids, but it's worth asking. From my perspective, it only takes a couple of seconds to say "Hey, I see Sabrina and Tommy (other friendly kids are playing pirates over there. I think they need a third pirate for their ship." Kids who are needing a little help and want to feel wanted will often rise to the occasion and go where their needs are being met, and in this way she can 'save face' leaving the group of girls that aren't nice with her pride intact. Letting them "Blame it on the teacher" is sometimes the nicest thing we teachers can do.:)