When rain is slated for the day you want to get a timely start for the ride so show up promptly. If you get a flat tire because your tires are worn out and the group gets soaked because the down time could have made it home or to the train dry, you will not be remembered fondly; present with a bike that has serviceable, properly inflated tires. Carry some rain gear or it will be like jumping out of a plane without a parachute. And, if you don't fancy a wet, brown streak down your back, put a rear fender on your bike. Lastly, don't dawdle, the ride is the thing. If you are lucky enough to get home dry you will have had to help make your own luck.