You can decide if you want to make all your members PRO automatically, or if you can sell it separately and let members pay for it. If you want to read more about upselling the PRO membership, check here.
When a member is not a PRO, there will be an option visible in their Mobile App menu with the option 'Become PRO'.
There's a default text in there, but you can easily edit this. To do this:
Go to Business Settings
Go to Mobile App
Go to Other Configurations