Check if submitted email address belongs to a user account

Question

How could I go about allowing a (not logged in) guest user to submit their email address, check behind the scenes if it belongs to a user account, and then return:

1. A login form if it does
2. A registration form if it doesn't

Answer 1

I assume you have basic knowledge about plugins and routing, if not I'll update my post in a few hours.. otherwise Your form could be

<!-- check if user submitted something -->
{% if userExists is defined %}
    {% if userExists == true %}
        <!-- user exists login -->
        hello {{ user.name }}

        <form method="post" accept-charset="UTF-8">
            {{ getCsrfInput() }}
            <input type="hidden" name="action" value="users/login">

            <h3><label for="loginName">Username or email</label></h3>
            <input id="loginName" type="text" name="loginName"
                   value="{{ craft.session.rememberedUsername }}">

            <h3><label for="password">Password</label></h3>
            <input id="password" type="password" name="password">

            <label>
                <input type="checkbox" name="rememberMe" value="1">
                Remember me
            </label>

            <input type="submit" value="Login">

            {% if errorMessage is defined %}
                <p>{{ errorMessage }}</p>
            {% endif %}
        </form>

        <p><a href="{{ url('forgotpassword') }}">Forget your password?</a></p>

    {% else %}
        <!-- user does not exist register -->
        <!-- snip see here for full content https://craftcms.com/docs/templating/user-registration-form -->

    {% endif %}

{% else %}
    <!-- parameter not set show search form -->
    <form method="post">
        <input type="hidden" name="action" value="test/checkEmail">
        <input type="text" name="email">
        <input type="submit">
    </form>
{% endif %}

And your controller

public function actionCheckEmail(){
    $email = craft()->request->getParam('email');
    if($user = craft()->users->getUserByEmail($email)){
        // user registered
        $userExists = true;
    }else{
        $userExists = false;
    }
    craft()->urlManager->setRouteVariables([
        'userExists'    => $userExists,
        'user'          => $user
    ]);
}