And you'll need a headset, but not an app.
http://www.nintendoworldreport.com/news/47501/fortnite-adding-party-chat-thursday
It won't take a fortnight for party chat to come to the Switch version of Fortnite.
During a demonstration on Treehouse Live today, Epic confirmed that the new Switch release will have party chat running starting "Thursday morning" (US time).
A standard cell phone headset will work for the chat, provided it can plug into the Switch's headphone jack.