To access and display a Yammer group feed in SharePoint you can refer the below article. But please make a note it would display a private group feed of Yammer.
Displaying Yammer feed (Private group) in a webpart...
The second approach is to display Yammer feed in SharePoint through JavaScript. You can use a Content Editor Web Part and embedd the code.
For your reference:
https://developer.yammer.com/v1.0/docs/embed
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