The Nintendo Switch Online app was scheduled to come out on July 21st but the app is now available for download for both iOS and Android Switch users. There isn’t much to see at the moment since the first game it supports, Splatoon 2, is coming out on Friday. When that launches, players will be able to use the app's voice chat feature with a special Splatoon hub. At the moment, though, it seems Nintendo is still working to stabilize the app's servers, which hopefully will help them avoid the fiasco during Pokèmon Go's launch. So don't be surprised if you can't log in yet. But when it does go fully live, you'll just need to sign in with your Nintendo account. 

Source: Polygon