I know that the master and slaves sites aren't sharing any table in the database, but can I share the users from the masters and the slaves with any bridge or something?

I think there are component that allow synchronise the users but I can not tell you more because I don't know those components.

User sharing is planned for june or july 2009.

if i use a component for the users sharing now can i replace it with the future version of jms?

i haven't found any component for user sharing.
I dropped 3 tables in the db, slavejos_user, slavejos_core_acl_aro and slavejos_core_acl_groups_aro_map
and created 3 new views with this sql query
CREATE VIEW slavejos_users AS SELECT * FROM jos_users

I got a problem now, the admin of one site is admin for all the sites, how can i put an admin as admin of only 1 site?

I see that you have used a SQL view that I expect to use in next JMS 1.2.x to share any component.
I think this is a good solution but as you have noticed, this means that all the users are shared to all the websites.
Therefore, the administrator of one website is also the administrator all the websites.
Normal as you have shared the users.
I don't see any solution except to create a special component that will provide the additional information that could be checked before Joomla.

i tried to check some component in joomla extensions but i haven't found anything. What do you think about a password before the backend login? I haven't any better idea atm