YES, with JMS 1.2.x, it is now possible to share the users between several website.
With this new feature, you can share the standard Joomla users but also a limited number of extension such as:

• VirtueMart users;
• Full VritueMart including the catalog, prices, ...
• Full Community Builder
• Full JomSocial
• Full SOBI2

This is not an exhaustive list.

Joomla multisite version 1.2.x also share the sessions (Single Sign-in). That means for example that you don't have to login again when you go from one website to another website.

In previous version JMS 1.0.x or JMS 1.1.x, it was NOT possible to share the users.