Switch will let you add friends via social networks and Nintendo Network ID at a later date

Coming from Nintendo...

Several methods can be used to add friends as quickly and easily as possible: · By linking Nintendo Switch to your Nintendo Account, your friends from Miitomo and Super Mario Run will appear as friend candidates so you can easily send them a friend request. Also, we are planning it so that friends from social networking services will be listed as candidates, too.

In the future, you will be able to send a friend request to friends associated with your NNID (Wii U/Miiverse) and Nintendo 3DS.

In some games you will be able to use an in-game interface to send friend requests.

You can send friend requests to those who have played with you recently.

You can establish a friend through local wireless communication.

You can also send a friend request by using a friend code.

from GoNintendo