You can follow other users if you are interested in what they are publishing. Actually you will see only activities on which you have the right to see it. For example, you won't see when someone commits s.th. on a private project.
To do so, visit the profile of the person you are interested in, by clicking on his name.
Then click on the star above the user activities.
Artifacts are everything, what is contributed by s.o.:
You can follow those, by clicking on the "subscribe" link on the top of those pages.