Build an array of checkboxes in drupal #drupal.

fapi_cb.md

Build up array of elements

  $query = db_select("users_roles", "ur")
             // TODO: fix hardcoded variable 5 for supervisor role
             ->condition("rid", 5)
             ->fields("ur", array("uid"));
  $result = $query->execute();

  $results_arr = array();
  foreach($result as $row) {
    $u = user_load($row->uid);
    $results_arr[$u->uid] = $u->name;
  }

next use that array results_arr as the #options in checkboxes element

  $form['supervisors'] = array(
    '#title' => 'Select Supervisors to add to Session',
    '#type' => 'checkboxes',
    '#options' => $resultsarr,
  );

in form submit callback grab the checked values

$sup_ids = array_filter($form_state['values']['supervisors']);