What do I need to use it? Is there an app?

Nothing to install from an app store. It runs in a modern web browser on a computer, tablet or phone. Your team's phone view, the crew app, can be added to the home screen so it opens like an app, and it keeps working with no signal. Your team signs in with their own email address and password. To email invoices from your own address you need an email account whose server details you can enter in Settings, and to take payments online you need your own Stripe, PayPal Business or GoCardless account.