Multi-site users

Hello,
I have WordPress 3.1.3 installed on a Linux server. I have multiste enabled and working, but I want users to be able to login to any of the sub sites with the same login, without having to register on each one.
So on multsite, when a user creates an account on ex1.com , How would I make it so that they can login to any of the ” sub” sites with the smae user name? so they login in at ex1.com/billy and ex1.com/Jane. Also, is there a way to make it so if they login on any of the “sub” sites, they stay loged in? So if billy logged in at ex1.com/bob , he will won’t have to login again at ex1.com , or ex1.com/jake. ? Thanks!