Nintendo has released its mobile app for the Nintendo Switch. It is available to download on Google Play and iTunes. The app is not currently functioning but it is expected to work on July 21, 2017 for the release of Splatoon 2. Download the app now so that you will be ready to use it.
Some interesting details from Nintendo’s website is that players are not able to start voice chat communication until they have joined the Online Lounge within Splatoon 2. The voice chat will disconnect while you’re talking on the phone or using another application, but your voice chat will restart in the same room once you open the Nintendo Switch Online application again as long as the game session is still ongoing.