If you go to a user's profile you will see a few profile menu buttons, such as Add to Friends, Send gifts, Send Private Message, Follow, etc. The Follow button allows you to follow a user's activities. If you do not want to add a user to your friends list, and do not want demonstrate your interest to them - you can simply click the Follow button to see their activities in the News Feed component on your Dashboard page.

You can also stop following this user by clicking the Unfollow button if you do not want to keep an eye on their activities anymore.

Furthermore, when you add a user as a friend, they are automatically added to your follow list, and you can decide whether to follow or unfollow them in future.