Hook into "add node to group"

Hi All,

I'm using Message + Flag, where users can subscribe to groups in order to receive email when new content added to group.
Any ideas how to hook into "node added to group" action? I've seen function tm_mymodule_group_content_presave(Drupal\group\Entity\GroupContent $entity), but it doesn't seem to work and looking at group.api.php there is only single hood defined.

Thanks