When you open your "phone" under the list of friends there are 3 buttons, I believe that they are FRIENDS, ADD, and INVITE. You click add then type in a username and it will send that person a friend...
REAL EASY!!! all you have to do is go to that person'a boutique,okay and near the comments it will say, add to favorites!
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It is said "Añádeme como tu amigo"(literal translation)
If your friends have a friend it is their mutual friend so you don't have to add them