After getting started with Yammer, the first thing you should do is join a group.
1. To join a group click the Browse Groups text, as indicated below.
2. The window below will popup. You will be presented with some suggested groups. Find the group you wish to join, then click the join button to the right of it.
If the group you want isn't in the list there is a link at the bottom ("View More Groups") which will present you with the full list of groups.
3. Once you have joined the group a notification will appear at the top of your screen indicating you have joined it.