As ride speeds increase, the riders will draft more, and ride closer together. So the slower groups are easier for beginners. They often are more careful to be a real no-drop ride, with rider counts or sweeper riders at the back.
Most ride leaders will welcome an email or call from a new rider. Find out: distance of the ride, typical flat road speeds, and perhaps average speed for the whole ride.
But average speeds really depend on the amount of climbing, and the number of stop signs or lights. For instance, I do a local ride that averages 15 mph most weeks, but has 3-4 longer hill climbs and a number of stop lights. So the 15 average includes typical flat road speeds of 18-20 mph (faster on downhills, slower on uphills, of course).
Some of the local rides are an out-and-back ride with easy navigation, or a route marked by spraypainted markings at each turn. These tend to break up into smaller groups, each riding it's own pace, since they don't need a ride leader to navigate.