Share the user accounts across multiple websites.
The Multi Site Users plugin will allow you to have multiple websites with the same user accounts giving you more flexibility to for example create subdomains for a store only website or a support only website or in case you different websites for each product but want to keep the users to be able to login across all websites. The component is prepared to work with the Joomla! users management and we can't confirm that works if you use other component to manage the users of your website.
You can use it by mixing websites from different domains and/or subdomains.
Users can be created and edited in both frontend and backend and will be syncronized in all websites.
Unlimited websites can be used, the structure is to have one master website and then one (or more) slaves.