Single membership, multiple sites

Hi, I would like to use a single membership database for multiple sites. So the users can be able to use one membership to use in all sites. Now to be more specific, (in case this is possible) my questions are as follows:
1) how can I do this with new installations?
2) is it possible to integrate this in existing sites?
3) is it possible to combine the existing membership information within different WP sites.

Thanks in advance,