Unless you have multiple Joist accounts, you should almost never have to log out of the app. In the unlikely event that you do need to log out of the app however, here's how:
iOS
Tap on More in the bottom right corner.
Then go to Settings, scroll down to the bottom and tap on Log Out
Android
Tap on Manage in the bottom right corner.
Then go to My Account and tap on Logout
Web
Click on your account name in the top right corner to bring up a menu.
Then select Logout from the menu.