Instructions: Join Yammer Groups
Clicking on Muni Groups (in the left menu) will take you to the Discover Groups view. Select a group you are interested in from this list and click on Join. When you join a private group, you will have to wait for your request to be approved by the group administrator.
If you know the name of the group you would like to join, you can use the Search field. As soon as Yammer finds the group you are looking for, you can join using the Join Group button in the top right corner. When you join a private group, you will have to wait for your request to be approved.