She's 12. At this age you just cannot get involved with calling the moms of other kids. It creates a lot more problems than it solves, it embarrasses your daughter, and it makes the other kids pull away from her! At this age, kids don't want their moms involved and they don't want to be doing things because another mom thinks it's a good idea. It makes them think there is something wrong with your daughter!
If your daughter wants to do something for Halloween, she needs to go to one of the groups and find out what time they are getting together. She can either ask to go along, or she can just act like she's part of the group and say "What time are we getting together?" She can ask them what they are dressing up as, and maybe ask their advice saying, "I can't decide between being a ___ or a ___. What do you think?" If they reject her, then she will know they are not her friends, and she can align herself with the other group, or strike out on her own with some new friends.
Either way, you just have to be less hands-on with regulating her social life. She can be a leader and not a follower, and take the initiative rather than sit around waiting. Others may mistake her waiting for a lack of interest - they want an enthusiastic member of the group, not someone who appears disinterested.