How do I use the #autocomplete_route_name property?
Users are entities so they're treated generically. The route is system.entity_autocomplete, with some parameters, but you don't need to get your hands that dirty. You can use the "entity autocomplete" form element type instead, which should be as simple as:

$form['project']['users'] = array(
  '#type' =&gt; 'entity_autocomplete',
  '#title' =&gt; t('choose users'),
  '#target_type' =&gt; 'user',
);


It's derived from a TextField, so you can use all the usual properties. Examples do seem to be a little hard to find other than in the core files (it's early days), but you can see all the available properties the entity_complete adds in EntityAutocomplete::getInfo(), and how they're used within other methods of the class.