How do I retrieve un-submitted values from a form and pass to another module?
First, you shouldn't have to modify the page callback from the user references module. Instead, use hook_form_alter() (or hook_form_FORM_ID_alter()) to change the #autocomlete_path attribute of the user reference text box to point to you're callback instead. That way you aren't modifying the module's stuff and you have the option of using the default implementation somewhere else. This is generally a better option than modifying another module's menu items.

For your particular use case, you'll do what you actually suggested; use the AJAX properties on the team fields to make the information available to the autocomplete callback. In practice it's slightly more complex than that, but you've already got the right idea. There's an article on Active LAMP that does pretty much the same thing.

First let's see about your custom autocomplete callback. You need to know which teams (Team X and Team Y as you put it) to filter by and what string to search for. In the following code, you first create a menu item for your callback, then define the actual autocomplete callback. The menu item uses two wild cards in the path to provide the ids for Team X and Team Y to the callback. You do not need to specify a wildcard for the search string, Drupal will handle that for you. In your callback, use the id information to narrow your user query results.

function trim_users_menu() {
  $item['trim-users/autocomplete/%/%'] = array(
    'page callback' =&gt; 'trim_users_autocomplete_callback',
    'page arguments' =&gt; array(2, 3),
    'type' =&gt; MENU_CALLBACK,
  );
  return $item;
}

function trim_users_autocomplete_callback($x, $y, $string) {
  // Here, use the query you altered for finding the correct users.
  $query = &lt;youre altered query&gt;;
  // Now, return the available User options.
  // Taken from user_reference_autocomlete, minus the hyperlink removal.
  $matches = array();
  foreach ($query-&gt;fetchAll() as $row) {
    $matches[$row['title'] . " [uid:$id]"] = '&lt;div class="reference-autocomplete"&gt;' . $suggestion . '&lt;/div&gt;';
  }

  // return for JS
  drupal_json_output($matches);
}


Now you need to alter the form to make the user reference field use this custom autocomplete callback. The url you use for the user reference field's #autocomplete_path will follow the pattern of the menu item you create. Suppose you want to filter the users to Team X, with id 1, and Team Y, with id 2. You're path will be trim-users/autocomplete/1/2. To get  team ids for the path, you use #ajax['callback'] settings on the Team fields to respond to change events. For example, set the #ajax['callback'] property to trim_users_team_selected. When the Team field value changes, Drupal will rebuild the form then run your #ajax['callback'] to update the form on the client. So, inhook_form_alter()`, check if the team fields have a value, and if they do use it to build the autocomplete callback path you assign to the user reference field.

function trim_users_form_alter(&amp;$form, &amp;$form_state) {
  // Check if it's the correct form, or use hook_form_FORM_ID_alter instead.
  ...

  // Check the $form_state to see if there's a value for the Team fields
  if (isset($form_state['input']['field_name']['#value']) {
    // Get the value of the field and use it in the autocomplete callback path
    ...
  }
  ...
}

// Return the User Reference field element from the rebuilt form. This will
// replace the existing field element on the client.
function trim_users_team_selected($form, $form_state) {
  return $form['user_reference_field'];
}


There's a few other things you have to do, like set the #ajax['wrapper'] property, but that's all part of using #ajax['callback'] so I'm not going to get into that here. This should at least get you on your way, you just need to figure out the specifics of retrieving and setting the form field values.