WordPress user accounts are unique to a single installation of WordPress. Even if they are hosted on the same server, there's no simple way (and, more importantly, probably no secure way) to hook them together.
WordPress accounts on WordPress .com can be added to other sites, but not accounts on self-hosted WordPress installations. The only "exception" to this situation is that a user account can be added to multiple sites if those sites are all part of a multisite network (which is essentially what WordPress.com is).
Note: Using the same email account to register user accounts on multiple sites will associate each user account with a gravatar account with the same email address, but that's pretty much only useful for getting a gravatar image or using JetPack's gravatar hovercard plugin.