If you go to find people you can see 'Add Friend Request' as a link on the side. Another way of adding a friend is going to the person's profile. If you go to any profile, you'll see the 'Add Friend Request' link underneath the person's profile picture. If you press the link that you'll be asked 'Are you sure you want to send a friend request to
'. If you press confirm, then you'll be sending an email to that person asking to confirm if that person is your friend.