Without the app? It's not clear. It may be possible on a game by game basis just like the Wii U.
Well, and don't know about party chat, but it seems that...
"After the free-trial period, most games will require a paid online service subscription from Nintendo in order to play online" in regards to the online service..."
"Nintendo Paid online service launching Fall 2017.. Free trial period starts in March."
"Our 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."
So, if I read correctly... some games can be played online as long as your a Subscriber with a Nintendo account regardless of paid or free version. Then, Online free trial services start in March, but they write
"This 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."
http://www.nintendo.com/switch/online-service/
After reading about 20 times it seems like it's more like this:
March:
Online gaming without app. No voice chat or lobbies since no app. No free games.
Summer:
Free, limited version of the app. So, maybe voice chat and lobbies? No free games.
Fall:
Paid subscribers get all online service, *free* limited time game, discounts/offers on digital games/content.
Paid app? Free App? Limited App? With Voice Chat and lobbies. Exclusive Deals: