Coming in 2018
This paid service lets Nintendo Switch owners enjoy online multiplayer gaming as well as a dedicated smartphone app that connects to your Nintendo Switch system and helps you connect with friends for online play sessions.
You’ll be able to play compatible co-op and competitive games online by signing in with your Nintendo Account. Online play will be free for Nintendo Account holders until the paid online service launches in 2018.
The service will cost $19.99 per year, and if you don’t want to commit to that much, it’ll run you $7.99 for three months or $3.99 for one month
Online lobby and voice chat - An all new dedicated smart device app will connect to Nintendo Switch and let you invite friends to play online, set play appointments, and chat with friends during online matches in compatible games─all from your smart device.
A free, limited version of this app will be available for download in summer 2017.
Also mentioned was subscribers will get to download a compilation of classic titles with added online play, such as Super Mario Bros. 3, Balloon Fight and Dr. Mario as well as special eShop offers that may include discounts on select digital games and content.
More details at the Switch Online Official Site.