Use the account for your organization. After you sign in online, you can install the app for quick access—if the connection drops while you're still signed in, you can often keep working with data on this device until you're back online. Don't rely on the app for fully offline use from scratch (you need the network to sign in and for cloud backup).
No account? Register your organization
Invited by a teammate? Open the join link your admin copied from Settings (don't use "Register your organization").