I've never noticed a plus sign... :lol:

When you click on someones name and go into their profile page there is a section on the bottom right that say's "Friends" in that area there is a link you can click on that say's befreind so and so. It will send a friend request to that person and they will show up as a friend in your buddy list on the main page, and it will add the + sign beside that persons name.