Any display of emotions, such as crying or fighting, can be a good opportunity to teach your toddler how to use words to express how she’s feeling: "Are you feeling frustrated because you're excited to use the slide and there’s a long line?" Walk her through what she’s feeling and why so that, eventually, she’ll have an easier time reacting verbally.
And that works both ways: When her playmates are struggling, tell your child the emotions her playmate might be feeling and why — especially if that child is upset as a consequence of your daughter's actions: “When you push Aiden, he feels sad.” This teaches empathy.
Also keep in mind that sharing is a concept that doesn’t come easily to toddlers; it can be too complex for them to understand. So don't be concerned when she takes her friend’s toy and announces that it belongs to her. "Taking turns" can be a better way to phrase the idea — but being patient and staying calm is also key.