I agree with Felt, and would add the following. Group riding involves three parts, two of which you can prepare for and one of which you can't. Part one is the physiological demands of group rides compared with solo rides. Group rides are going to be a lot more hard/easy than solo rides. This is because little gaps will open up and riders will go hard to close the gap to retain the draft, creating an accordion effect that ripples through the group. You can practice this by doing your solo rides with hard/easy efforts rather than a steady state effort. Part two is riding position. You want to absolutely avoid overlapping your front wheel with the rider's rear wheel in front of you. It is your responsibility to stay clear of his wheel and not his responsibility to look back before cutting left or right and if your wheels tap you will go down instantly. I also suggest to new riders who ride in my groups to use their rear brake first or simultaneously with the front brake because the rider behind you can see it and anticipate that you're braking. If you use your front brake first or only, you're more likely to catch the rider behind you off-guard and if he's close to your wheel he could end up in an overlap quickly. Part three is the group etiquette with the specific group you ride with. You can't prepare in advance for this, and the unwritten rules can vary all of the map. Some groups do a lot of talking, some hardly talk. Some groups don't want you to stay on front more than a minute or two, and some groups let you stay on front as long as you want because they understand training and that you may be trying to get in an aerobic effort. You'll just have to watch and learn when you join a group.
My final suggestion is to avoid one of my pet peeves in group riding, and unfortunately it is very common. If you're on front and you see an obstacle large enough to take down a bike (e.g., a large rock, a tree branch, or anything else that could take down a rider), steer the group well to the side of the obstacle and point it out. If you simply ride by the obstacle a foot or two to the side and point down at it as you go by (as many riders do), there's a good chance somebody behind you will go down. If they're right behind you and offset a bit to the side of the obstacle, you're blocking their forward vision and by the time you alert them to the obstacle by pointing at it as you ride by, they're committed to their line and can't do anything to avoid it. This happened on a ride I was on and a guy went down and broke his femur. Totally unnecessary.