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)
hook_group_content_update(Drupal\Core\Entity\ContentEntityInterface $entity)

But neither seem to work and looking at group.api.php there is only single hood defined.

Any advice would be helpful.

Thanks