You are required to enter your PIN before you can send money to someone on your contact list -
You can now send money to anyone else who has a Monzo account, on iOS or Android. It’s sent instantly, securely authenticated with your PIN and of course, it’s totally free.
Obviously you should secure your phone with a passcode & if you’re looking to stop others from accessing the app, once they have your phone then you can use one of the apps mentioned here -