Burning Justice
the superior princess
If you select it that friend will be bumped to the top of your friend list. That's all it does. You can't send messages on the Switch by the way.
You have the option of making it so only best friends can see your online status, so if you only want certain people from your friends list joining you online, that's the way to do it.
EDIT: That feature is not new, by the way.