Yammer notifications are not implemented in Grafana, by default. So I decided to develop a little gateway that publish Grafana alarm webhooks to Yammer thread.
Add Webhook notification channel to Grafana configuration pointing to gateway url (with following format). Check include image.
In your Yammer configuration panel, add a new custom app. Then generate development token. Copy/past it in env.txt.
http://<grafana-yammer-gw>:<port>/publish/<yammer-group-id>
docker build -t ziouf/grafana-yammer-gw -f dockerfile -q --rm .
docker run -d --env-file env.txt -p 80:80 ziouf/grafana-yammer-gw