Hi, C.. Oooooh, she's jealous! She's not used to being around other children, and she's afraid the other kids will take her toys as well as take up all the attention. That's why she's acting out. I think that to get her feeling more secure about having other children around, you and other adults could sit on the floor with them all, and make sure that all the children play with you and each other. Make sure that they all get about the same amount of attention. It doesn't sound like your daughter is verbal enough yet to understand the command to share; you're going to have to demonstrate it, as well as demonstrate that she isn't going to lose anything by being around other kids.
Some kids are naturally more social than others; some need more time to get used to the idea that they are not the center of the universe. ( : Maybe you can show her pictures of when she was smaller, and let her know that she's grown a lot and is a big girl now...and big girls play nice, or some other positive message. Make sure you model playing nicely with others. Start at home by playing nicely with each other, and build on those skills a little bit at a time.
You might also try having play dates with fewer children for her to be jealous of.
Peace,
Syl