To find the feed ID for a Yammer group, look in the address bar of your browser.
Sign in to Yammer and go to the group page you want.
With the group page open on your browser, look in your browser's address bar at the page URL. At the end of the URL, find the numbers after “feedId=” - there are usually six or seven digits. Those numbers are the Yammer group's feed ID.
In this example above, the group's feed ID is 504583.