您可以通过将此代码添加到bp-custom.php 或主题的functions.php 文件

add_filter(\'bp_core_signup_send_validation_email_message\', \'add_username_to_activation_email\',10,3);

function add_username_to_activation_email($msg, $u_id, $activation_url) {
    $username = $_POST[\'signup_username\'];
    $msg .= sprintf( __("After successful activation, you can log in using your username (%1\\$s) along with password you choose during registration process.", \'textdomain\'), $username);
    return $msg;
}

EDIT: 没错，如果用户在用户名中使用了空格，Buddypress不会向他显示错误，注册过程将继续。下次用户尝试使用用户名和空格登录时，登录过程将失败。所以我们必须更换$username = $_POST[\'signup_username\'] 具体如下：

add_filter(\'bp_core_signup_send_validation_email_message\', \'add_username_to_activation_email\',10,3);

function add_username_to_activation_email($msg, $u_id, $activation_url) {
    // $username = $_POST[\'signup_username\'];
    $userinfo = get_userdata($u_id);
    $username = $userinfo->user_login;
    $msg .= sprintf( __("After successful activation, you can log in using your username (%1\\$s) along with password you choose during registration process.", \'textdomain\'), $username);
    return $msg;
}