The more you ride, whether alone or with a group, the easier riding itself will be. The easier it will be to lift a hand off the handlebars for instance. I remember when I wasn't comfortable grabbing my water bottle and drinking during a ride and, worse yet, putting it back in the bottle cage. One time my husband made a comment about drinking, I think reminding me to take a drink so I don't get dehydrated. I told him, "I don't' drink and ride!" I used to only drink when stopped at a signal or for a break. But that all changed. I routinely grab tissues from my back pocket and blow my nose one handed and put the tissue back. I'm quite the expert at that technique!
So, ride, ride, ride, and you'll get more comfortable with the actual act of riding. When you are more comfortable handling your bike, it will help your comfort in a group.
Being around other more experienced riders can teach you a lot, too. I agree to keep an eye out for riders that you feel comfortable being behind. Maybe even buddy up with someone and make it known to that person and others that you would like to stay behind them specifically. You should let others know that you are a little nervous. They can then help you out with tips and/or just watch out for you in various ways. Most cyclists are ultra friendly and willing to help each other out. It's one of the things I like about the sport and riding with others.
I'm familiar with Fullerton, though it's one of the few places in So. Cal I think I've never ridden. Wait, I take that back. I know I rode through Diamond Bar and Brea to get to the Santa Ana bike trail. We must have gone through part of Fullerton. Anyway, I live in Upland, having just moved a few months ago from Rancho Cucamonga. I lived in OC growing up (Huntington Beach), and have also lived in Orange, Anaheim hills, Anaheim and Brea. But I didn't start seriously riding until I lived in Rancho Cucamonga. If you haven't ridden the Santa Ana river trail, you really should. It's one of my favorite rides. A great one that involves no stopping, unless you want a break. And no cars!
I have often ridden it from Yorba Linda Regional Park. It's 44 miles to the beach and back to that point. But there are many places to get on the trail.
Speaking of riding in traffic with no bike lanes, though, this is something you'll get more used to in time as well. My husband has taught me that when I'm in the bike lane, to actually ride closer to the traffic than to the right side of the lane, because most of the debris is further to the right. I have also found that cars often give riders far more room than necessary and, while we should all be cautious of cars, in time it becomes less scary to ride along side them, bike lane or no bike lane.
Also, when you are in a group, the cars are far more aware of your presence than if you are riding alone. You are less likely to be hit when in a group.
Finally, if it's possible to find a smallish group that rides in areas you are more comfortable with for a while, I recommend you do that. Our club has a women only ride on Sundays and it's the perfect place for newer riders to get used to riding in a group. They don't ride in major traffic and are very unintimidating over all. Hey, if you want to drive up to RC, you can do that ride any time.(My club is Cycling Connection. http://www.cyclingconnection.org/ )