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Why would I want to allow multiple accounts to be associated with one email address?

Maybe your site is one that doesn't mind if users can sign up for multiple accounts from the same email address, maybe for different identities. More likely, you as an admin, plugin developer, and/or theme developer would like to be able to create multiple accounts on a blog to test various permissions or just want to test the blog having numerous users and don't want to have to assign unique email addresses for each account.

Can I limit who can create multiple accounts for an email?

Yes. You can specify a limit on how many accounts can be created per email address. You can also explicitly list the email addresses which are allowed to create multiple accounts (useful for just allowing admins to have multiple accounts).

How does the plugin affect the "Lost your password?" feature?

The clearest method for resetting a forgotten password is to supply the username on the "Lost your password" form when prompted.

If an email address is instead supplied on the form, WordPress will send an email to that address with reset information for the first account found associated with that address. If multiple accounts are associated with that email address, then the email will include a listing of all associated usernames. In order to reset the password for a specific account, go back to the forgotten password form and supply the desired username, or if the email that was sent happens to be for the account that needs the password reset, follow the instructions and link in the email. Bear in mind that the password reset email can be safely disregarded if it relates to an account that shouldn't be reset.

What if I allowed email addresses to create up to 5 accounts and some people did so. Then I lowered the limit to 2. What happens now that some email accounts exceed the current limit?

Nothing happens. The plugin does not do anything with existing accounts. Those email addresses will not be able to create new accounts because they exceed the current limits.

Is this Multisite compatible?

Yes.

Is this BuddyPress compatible?

Yes, for at least BuddyPress 1.2+ and 1.3+, and perhaps other versions.

In Multisite, why do I get this error message when trying to register for another account with an already used email address: "That email address has already been used. Please check your inbox for an activation email. It will become available in a couple of days if you do nothing."

If you're seeing that error then it means the email address used for the new registration matches one used by an account in the signups table. Basically, an account has been registered with that email address but have not been activated yet. Only one account can be in this registered-but-not-activated state per email address.

Before that email address can be used for another account, you have to activate that pending account, delete the pending account from the signups table, or wait a couple of days until the pending account expires.

Why do I see this notice in my admin: "NOTE: Allow Multiple Accounts is not able to function as intended because another plugin has overridden the WordPress function `get_user_by()`"

You are using another plugin that has overridden WordPress's get_user_by() function, which this plugin needs to do itself in order to function as intended. (This is necessary due to the lack of any more direct methods within WordPress for achieving its goals.) You will have to find and disable the other plugin, or disable this plugin.