There is no easy way for multiple people to work on the same app. However, this is a common question so this article will outline two possibilities:
This is the easiest way to work on the same app with others. But note, you can't work on the app at the same time (i.e. 2+ people can't work in the app builder for the same app at the same time).
An "App Community" is a place where you can publish your app; other community members can create a duplicate of the app. The workflow for this option mirrors what most software developers do:
Learn how to make your own app community.