wordpress form with roster registration of users

I’m trying to build a manual form on a WordPress page.

My goal is to create a dropdown menu of all the users with a fixed role (in my case “athlete”) and register single users in some sport events.

My code:

<form name="team_captain" method="POST" >

  Carnival Name: <input type="text" id="carnival_name" name="carnival_name" />
  <input type="submit" name="submit_carnival" value="Create Carnival"/><br />

  Event Name: <select name="event_name">
    <option value="Ironman Race">Ironman Race</option>
    <option value="Board Race">Board Race</option>
    <option value="Ski Race">Ski Race</option>
    <option value="Surf Race">Surf Race</option>
  </select><br />

  Athlete Name: 
  <select name="athlete_name">[insert_php] echo getUsersByRole('athlete','athlete-selector'); [/insert_php]</select><br />

  Age Group: <select name="age_group">
    <option value="Under 15">Under 15</option>
    <option value="Under 17">Under 17</option>
    <option value="Under 19">Under 19</option>
    <option value="Open">Open</option>
  </select><br />

  Sex: <input type="radio" name="athlete_sex" value="Male">Male <input type="radio" name="athlete_sex" value="Female">Female<br />

  <input type="submit" value="Insert Race"/>
</form>

[insert_php]
  if(isset($_POST['submit_carnival'])) {
    $carnival_name = $_POST['carnival_name']; 
    echo "Registrations for ".$carnival_name;
  }

  elseif(isset($_POST['submit'])) {
    $event_name = $_POST["event_name"];
    $athlete_sex = $_POST["athlete_sex"];
    $age_group = $_POST["age_group"];
    $athlete_name = $_POST["athlete_name"];
    echo $event_name;
    echo $age_group;
    echo $athlete_sex;
    echo $athtlete_name;
  }
[/insert_php]

At this point I can display the name of the carnival, but I can’t display the instances of the athletes. And the dropdown list of the athletes doesn’t work.

This is the function:

function getUsersByRole($role,$name,$selected = '',$extra = '') {
global $wpdb;

  $wp_user_search = new WP_User_Query(array("role"=> $role));
     $role_data = $wp_user_search->get_results();
foreach($role_data  as $item){
$role_data_ids[] = $item->ID;
}

  $ids = implode(',', $role_data_ids);
  $r = $wpdb->get_results("SELECT *   from ".$wpdb->prefix . "users where id IN(".$ids .")", ARRAY_A);

  $content .='<select name="'.$name.'" id="'.$name.'" '.$extra.'>';

  if($selected == ''){
$content .='<option value="" selected="selected">Choose a user</option>';
}else{
$r_user = $wpdb->get_results("SELECT *   from ".$wpdb->prefix . "users where ID = ".$selected."", ARRAY_A);
$content .='<option value="'.$selected.'" selected="selected">'.stripslashes($r_user[0]['display_name']).'</option>';
}

  for($i=0; $i<count($r); $i++){      
  $content .='<option value="'.$r[$i]['ID'].'">'.stripslashes($r[$i]['display_name']).'</option>';
  }
  $content .='</select>';

  return $content;
}