Group riding will teach you a lot about surviving a crit racing. You can do a USAC single day license, as pointed above, to enter your first race. As STP said, people in these groups likely race, so they’ll be able to get you in the fold.
Training series / races are a good way to go, but no less serious. Our local Wedneday night training series is selling out Cat4/5 at 75 riders and almost selling out Cat1-3. Lots of heavy hitters and teams in all of them. The front peloton in last night’s Cat4/5 was averaging 27-28mph on a rolling course with 10mph cross / head winds in spots. Group skills will teach you a lot how to ride in that environment because when you’re going that fast, with that wind, there is no coming back if you get caught out in the open. Also, not matter how good of shape you think you are, racing is still humbling… people are baller. Just keep at it and you’ll be up in the mix eventually. There are skills (mental, reading the group, etc) that you just don’t get anywhere but racing, so that’s a learning curve too.