On a Macbook Pro running Snow Leopard, while one user is using the laptop, is it possible for another user to remotely log in and user the laptop at the same time in their own session?
Actually that is for OS X Tiger, you could use Teamviwer it offers loads of features, go ahead and download it at http://teamviewer.com/
Mac OS X doesn't have a native Terminal Services-like solution.
This page claims to have a solution to let multiple people use the Mac simultaneously. It requires people to already be logged in with Fast User Switching, then allows other users to VNC into their hidden sessions. I haven't tested this out and the website has no mention of Leopard or Snow Leopard, so I don't know for sure how well it works.