It's likely that there will be some groups that you care a lot about and others where you might have less interest, perhaps because they are less relevant to your everyday work. Because of this you might want to limit the groups that appear in your Radar activity stream.
To do this click on the drop-down menu button at the top right of the page and choose the My preferences option and then select the Activity stream page. You can now choose which of the groups you want to see.
Note that the My stream section on your Radar will still show how many items are unread for groups that you have opted not to see in the default activity stream.