I have not tested this, for more information see https://codex.wordpress.org/Function_Reference/get_users
add_filter('acf/load_field/name=user_groups', 'populateUserGroups');
function populateUserGroups( $field )
{
// reset choices
$field['choices'] = array();
$users = get_users();
foreach ($users as $user) {
$field['choices'][ $user->ID ] = $user->display_name;
}
return $field;
}