How to sync domain user desktops to be accessed from anywhere?

Is there any WordPress plugin that can be used for - registering a user on domain [main directory of root] and with which user can get registered on every other sub-domain too [sub directories in main directory] and can comment and do stuff?

• It is not a Multisite interface that I am talking about, as in that case there are many plugins. In this case there are different wp-admin for different sub domains [sub-directories] like "http://a.com/b", "http://a.com/c" User should only register on http://a.com and he should be able to post comments and stuff on any sub domain including main domain.

• Answer:

  I would set this up as having each subdomain use the same wp-users table in your database.