Sub-domain's database setting

I recently create a sub-domain for my WPMU site, I want to install wp 3.0 to this sub-domain. But, I have a question about which database should I use for this sub-domain.

Should I use the same database of my main WPMU site’s database?

Or should I use a new database separately?

Because I want every member who register with my main WPMU site have the authority to login to my sub-domain, if this is possible, what should I do to get it done?