Find a local shop that sponsors a team or has group rides. Also, contact USA Cycling and see if there is a Local Association (New England has one for example). The LA might have bike handling clinics or something like that to offer new racers. On a different forum someone was talking about setting up a Cat 5 clinic to help them learn enough to get to Cat 4.
No matter what you do, upgrading from Cat 5 to 4 is NOT about fitness. It is about your ability to ride safely in a field. A lot of very fit (probably as fit as a Cat 2) racers don't understand this and it's very difficult to explain to them that just because you're fast doesn't mean you can race. In fact, upgrading from Cat 4 to Cat 3 is also not about fitness. A hint - if you can upgrade based on experience, then the upgrade is concerned with bike handling, not with fitness. The first fitness related upgrade is when you go from Cat 3 to Cat 2 - then only race places count, not experience. Cat 2s are limited to those who are both good bike handlers *and* fast. Cat 3s and below are essentially for various levesl of bike handlers who may or may not be fit.
For example, if you jumped into a leading F1 car right now (Ferrari or McLaren, say) and you started an F1 race, from a car point of view (i.e. "fitness"😉, you'd be as fast or faster than anyone out there on the track (way faster than the back of the field cars). From an experience point of view...well, you may not make it through the first turn of the race.
You will absolutely have to ride in a group before you race, and ideally you should be doing bike handling drills (bumping, touching wheels, one hand, turn around without veering, wiping tires) if at all possible. Bike handling drills are typically done with other riders since you can't bump or touch wheels without a rider to bump or touch wheels.
Using a race to learn how to ride in a group is not a good idea, either for you or for those around you.
cdr