... but it isn't working yet (bummer). 

[EDIT] It looks like it's working now. 

[EDIT2] Since the app is operational and we're just few hours from the Splatoon 2 release, I've added the poll.

Links below. 

AppStore

Google Play

Windows Phone Store

Description

The Nintendo Switch Online app is designed to help enhance your online gameplay experience on Nintendo Switch™. With this app, you can check game-specific services (e.g., SplatNet 2 for Splatoon 2), invite friends to play with you via social media, and use voice chat while gaming.

◆ Do you play Splatoon 2? If so, you can make use of the dedicated SplatNet 2 service!

You can use SplatNet 2 to check all sorts of vital battle-related information, including match results, stages, and rankings! 

◆ Inviting friends via social media

You can invite friends to play with you in supported games via social media such as Facebook and Twitter. Of course, you can easily invite your Nintendo Switch friends, too!

◆ Using voice chat while gaming

You can enjoy voice chat in different ways depending on the type of game you're playing. Chat with everyone in your room or split voice chat into teams and have it out head-to-head!

*In order to use these services, a Nintendo Switch game compatible with this app is required. Splatoon 2 is compatible with this app.


Attention:
● You can use the Nintendo Switch Online app for free until the paid subscription service launches in 2018.
● A persistent Internet connection is required to use this app. Cellular data may be used.
● A Nintendo Switch console, a Nintendo Account, and supported game are required.
● You must be at least 13 years or older to use this app.
● Your use of this app is subject to the Nintendo Account User Agreement, available at support.nintendo.com.

The rating comments are hilarious^^

One side giving 1* because the app exists and the other side is complaining about the 1* reviews while giving 5* to an app that isn't even working right now^^

So basically Metacritic User Reviews

I don't why people are complaining so much about the fact that it is on mobile. The Switch is a portable machine but it doesn't have a constant internet connection outside. It seems pretty logical to me.

The question would be why a mobile device in 2017 does not have a constant online connection.

Battery life, probably.

Windows Phone user

That's like buying a Vita and bitching about Mario and Pokemon not releasing on it

I wonder how they solved that issue in other mobile devices. They must have some weird contraption to turn off mobile connections when they're not needed.

Really glad the Switch doesn't have 3g/4g internally to be honest, takes 5 seconds to turn my phone into a hot spot for the system and tether it to the net on my data plan, costs me nothing. Who doesn't have a phone capable of doing this in 2017?

The Vita had a 3g version of the system... that did wonders for it.

Erm, if you don't have access to an internet connection for voice chat with the Switch, how will you play online multiplayer at all? This is a fairly strange argument.

Honestly, there's really no excuse for a portable tablet device not having more options when it comes to online play in 2017, there really just isn't.