At launch, the only game compatible with the Nintendo Switch Online app will be Splatoon 2. The app will let users voice chat with friends, invite people to online matches, create teams, and access SplatNet 2. SplatNet2 offers up online play statistics and info on upcoming game features.
The debut of the app means Splatoon 2 and other games are unlikely to have built-in voice chat capabilities, instead requiring the separate app, but Nintendo has been promoting an upcoming headset accessory from Hori that will combine audio from the iPhone and the Nintendo Switch for a more seamless play experience.
Nintendo Switch Online for iOS will let you invite friends with Splatoon 2 to Private Battles, League Battles, and Splatfest Battles across social media and "other services," and it will support push notifications for instant alerts when an invite is received.
Though the app will launch with limited functionality, it will expand to encompass other games as Nintendo builds out its Nintendo Switch Online service.
The app's functionality and the Splatoon 2 online play will be available for free until the full Nintendo Switch Online service launches in 2018. Following the launch of Nintendo Switch Online, most games with online gameplay will require an ongoing subscription. Nintendo Switch Online will cost $19.99 per year or $3.99 per month with a monthly subscription.