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You can search for friends and request that someone be your friend by inputting their Friend ID.
Follow the below steps to send a friend request.
1. Tap Menu on the main screen, then tap Friends.
2. Tap Add Friend in the bottom left of the screen.
3. Input the ID of the person you want to be friends with, or send a request from your list of Nintendo Account Friends.
By becoming friends with people from around the world, you can do the following.
・Compete with each other via Friend Rankings
・Play multiplayer together
・Send Greeting Coins to each other
For more information about Greeting Coins, please see the article entitled "What are Greeting Coins?".
You can add up to 49 friends.