How do I add and watch my friends' activity?

There are a couple of ways to do this.

1) Profile Page.
When you see a submission that you really like, click on the name of the person who submitted it. This will take you to their profile page. Next, click on the “Add as a Friend” on the left side of the page.

2) Content Page.
Click on the “Send Friend Request” below the submitter’s name on the upper right side of the submission’s detail page.

Once added, you can track their activity in the "Networks’ Activity" tab of your profile. If they confirm that they actually are friends with you, they will show up on your Friends list.