Remove her from the others so she cannot behave like this, sir her on a chair away from them and tell her to stay there till she stops, if she gets off the chiar quietly put her back, over and over if you have to, she needs to be given Bounderys regarding hitting others, do not let her with them to play till she stops this behavior also try to figure out and ask her why she does it is there some jealousy there, are they younger than her and she feels left out and acts like this to get attention.
Get a professional to observe her. Go with your gut as you know your child best. Involve her in more social activites, stick to your guns, consistancy is key. Dont just wait for her to out grow this, as it may get worse.