Yes, you can!
On your QuitNet profile there is a button that says 'User search' -- you can click this to search for another member by their first name or nickname. All member matches will be presented to you and you can click the 'follow' button to begin following a member.
If you know a friend's email and want to find their profile quickly, the best way to do that is to send them an invitation via the page below. You can also get there under Settings > Tools and Resources > Invite
When you enter their email it will pull up their profile and give you the option to start following them.