What grinder do you recommend for the PRO2?

It is imperative that you use a high quality, espresso-capable burr grinder paired with freshly roasted, whole beans ground to size right before use.

The PRO2 portafilter is "non-pressurized", which will allow you to craft true espresso, and therefore requires use of a very fine, consistent grind to build resistance in the brew head to create pressure. You must have an espresso-capable grinder to take your shot into your own hands and find the perfect size for your specific roast, dose, bean freshness, etc. 

Note that pre-ground coffee or coffee ground from your local shop will not work, as it will not be "dialed-in" to your specific variables. Additionally, you won't achieve a beautiful crema, as the CO2 that creates it begins to escape once ground. 

If you're looking for a grinder recommendation, we of course recommend our Royal Grinder - learn more here. If looking at alternatives, ensure it has been vetted for use with espresso, has high-quality burrs, and at least 60 adjustment settings. Most capable manual grinders will be $150+, with electric options being more.