This is about introducing a dog to a harness, but you can absolutely use the same steps with a collar. Keeping things very slow and positive is key - try not to push your puppy past a step until she's ready and super confident and relaxed.
I'd definitely take a break with puppy playdates until you work this out. If she's spending the whole time stressed about wearing a collar, she's not learning anything good about being around other dogs and puppies. At best, she might just not be paying enough attention to really get much out of it. At worse, she could be learning that she has to wear uncomfortable things around other dogs, and so other dogs mean uncomfortable things happen to her. That's not the greatest lesson for her, as you might imagine.