You can decide if you want to make all your members PRO automatically, or if you want to make it part of your membership 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 standard text in there, but you can easily adjust this text the following way:
Go to Business Settings
Go to Mobile App
Go to Other Configurations
Adjust the text