email wordpress users using BCC

First: Don’t use mail(). Use wp_mail() instead.

wp_mail( 
    // Send it to yourself
    get_option( 'admin_email' ), 
    'Your subject', 
    'Your message', 
    // extra headers
    array (
        'Bcc:' . implode( ",", $usersarray ),
        'From:' . get_option( 'admin_email' )
    ) 
);