You might put this code in functions.php
:
$GLOBALS['allowed_users']= ['mikejordan', 'jamesbrown', ...];
add_action('user_register','my_function');
function my_function($user_id){
// get user data
$user_info = get_userdata($user_id);
if ( in_array($user_info->login, $GLOBALS['allowed_users'] ) )
{
$res = $wpdb->query($wpdb->prepare("SELECT activation_key from {$wpdb->signups} WHERE user_login = %s)", $user_info->login));
if (!empty($res)) {
wpmu_activate_signup( $res->activation_key );
}
}
}