K.D.
All I can ever think about when I think of this is the time I didn't invite the new girl at work to my bridal shower (I didn't feel that it was very nice of me to ask a gift of someone that I didn't really know! I heard second hand that she was sad not to be invited).
But honestly, I think you should invite the new girls. They are so young that there is no way that they will have trouble feeling included, I think its a good idea to invite them.