From a stand-alone activity click Follow this Activity from within an activity.
From a community activity, click Following Actions > Follow this Activity.
You can also track events in the activities that you are following from the home page by clicking I’m Following in the navigation.
If you no longer want to receive notifications about the latest updates to an activity, you can stop following it at any time. To stop following a stand-alone activity, click Stop Following this Activity. To stop following a community activity, open the community activity and click Following Actions > Stop Following this Activity.