The game allows players to have a partner.
You can allow another player to access your player, train your horses or create tack by adding him/her as a partner.
The access of your partner is limited, in order to protect you.
Your partner will not be able to send letters in your name, it won't be able to access your 'bank' account and it won't be able to sell your belongings in the shop.
Your partner does not have to know your password
The whole thing is made to protect you from other people who want to scam you. However before you add someone as a partner, make sure that person is trustworthy!

A partner is someone who can play with your player
The list of friends (that you will see) is the list of players that you can play for.

When someone adds you as her/his partner, that player will appear in your friends list. Click on the name, and you become that person.
To add someone as a partner, click on Partner, then look on the menu. Type the name of the player and click on the Add partner link.