To send a friend request on SA.
Visit the profile page of the person you wish to add as a friend and click the add to friends button. Your invitation will appear on that person's page waiting for them to accept or decline it when they next login.
Some people will accept every friend request while others may decline for whatever reason, don't be offended if that happens there are thousands of people that are members here and many more join each day. Some people prefer not to have any friends as is their right.