niobi-8.x-2.0-alpha4/modules/niobi_group/niobi_group.routing.yml

niobi_group.group_admin:
  path: '/group/{group}/admin'
  defaults:
    _title: 'Admin Settings'
    _controller: '\Drupal\niobi_group\Controller\NiobiGroupController::settings_page'
  requirements:
    _group_permission: 'administer group'
    _group_member: 'TRUE'