There are two main screens in Pronto - the drawer screen (on the left) and the chat screen (on the right).
Inside the drawer are categories and groups. Think of categories as folders that hold groups inside them.
How to navigate between the chat and the drawer
Select a group from the drawer to enter into its chat screen. If you are on the chat screen but want to go back to the drawer, tap the arrow at the top left or simply swipe on the screen to go back.
The layout is displayed in a similar way in the web/desktop version, though the drawer will always remain visible on the left.