How to: Allow Multiple Accounts with the Same Email Address
So you would like to allow multiple user accounts to share an e-mail address in WordPress? Allow Multiple Accounts for WordPress is just the plugin for the job. It lets your visitors create and register accounts using the same e-mail address. Webmasters have the option to enable this feature for only certain accounts.
In many forums and online communities, using multiple accounts and e-mail addresses is forbidden and for good reasons. Nevertheless, this plugin lets you make this option available to your members. In fact, you can specify a limit to the number of accounts an e-mail address can have.
Webmasters will be able to see user accounts that share e-mail addresses in the backend. This is a free plugin.