Can you be a little more specific, please?
Are you trying to track the position of your own avatar, or someone else's?
If you are tying an Avatar to the ViewPlatform, then you should be able to get the current position from the TransformGroup of the ViewPlatform, by extracting the Transform3D and pulling out the translation portion.
If you are tracking other people's avatars then you will most likely need to keep them in a list somewhere if you are updating your view.
Or is this more of a networking question - like how can you get notified when other people move their avatars around?