Hi CG and welcome. Being de-lurked is never a bad thing.
if you feel confident in the group rides from a close quarters/bike handling perspective go for it. It will give you a taste of actual racing, you'll get to share in the splendor that is Offthebackistan, and join the fraternity of people who have pinned on race numbers upside down.
From a prep perspective I would train for the durations of the climbs, that's where the passports usually get stamped. Three weeks is not a lot of time but that's less of an issue for a first race then it is once you start planning a racing season after you have some tome under your belt.
As everyone will verify, racing and group rides are different animals, especially when you are spotting folks 4 decades, so keep expectations low and just have fun. Be sure to report back.