Custom route using access check error with arguments
Digging deeper into what causes the error message brought me to the drupal docs about Parameter upcasting in routes. When passing entity parameters to an access check function you need to upcast your parameter and tell Drupal of which entity type your argument is. I.e. in my_mod.routing.yml add options and argument type:

mymod.add:
  path: '/mymod/add/{group}/{plugin_id}'
  defaults:
    _controller: '\Drupal\mymod\Controller\MymodController::addNews'
    _title: 'Add news'
  requirements:
    _group_content_create_access: 'TRUE'
  options:
    parameters:
      group:
        type: entity:group