She'll very likely outgrow some of this. A lot of her excitement, right now, is "puppyhood driven". Just know that for right now, when you see someone approaching you'll need to get your hands on her to get her to focus & listen to you. Give a little tug and say what you say to her to get her to stop doing something. Then when you tell her to sit, you'll probably have to actually hold her and help her to 'remember' what sit is. Then, like yorkiepuppie said, ask the other person if it's ok for her to greet them. Try to keep her somewhat under control, don't let her just explode all over them with a greeting.
She'll start to realize that sitting quietly will get her what she wants quicker and that jumping and acting crazy is not acceptable behavior. But she is still a puppy and has that wonderful puppy outlook and enthusiasm, just be firm, consistent and give her some more time to grow up.