The fact that she enjoys kindegarten is great. Please don't force her to interact if she chooses not to, that will likely only make her uncomfortable and cause her to retreat further. It may be good to try and get her around only one person at a time and see how she does. Sometimes being in big groups can be intimidating, so if she can make one good friend, that may make it easier for her to open up to others.
Take it slow, don't force the issue, and raise her confidence by supporting her through this transition into elementary school!
Boys Town National Hotline-A Resource for Parents and Teens