Knowledgebase
How to enable and disable customer account sharing in a multistore in Magento
By default, account sharing in Magento is set on a per website level. This means that if you have created a multistore with several websites, a customer account can be used only on the website within the multistore for which the account was created. The customer won't be able to log in using that account on the other websites.
If you want to, you can make account sharing global. In this way a single customer account can be used to log in to any of the websites within the multistore no matter on which site the customer has registered. To do this log in to the admin panel of your Magento, go to System menu>Configuration>Customer Configuration button in the Customers section on the left>Account Sharing Options panel on the right. Set the Share Customer Accounts drop-down menu to Global and click on the Save Config button.
For information on creating a multistore check out the article on Magento multistore.