If you have friends from other apps linked to your Nintendo Account who also play Animal Crossing: Pocket Camp, you can become friends via the following procedures.
① Select Social
② Select Friends
③ Select Search
④ Select Search by account.
Friends who are playing Animal Crossing: Pocket Camp will be displayed.
Tap their name to send a friend request.
When the friend request is approved, you will become friends.
* If friends won't display even after you've tried the above steps, please check out the question below.