Riding the trails when wet causes serious damage and more work for everyone, so we encourage all trail users to not ride wet trails. If there is anytime you're not sure, test ride some of the trail first. If you start seeing tracks, you've already damaged the trail, and that's when you need to stop.
If parts of the trail are rideable and some are not, either 1) ride through the wet stuff - this keeps the trail from getting wider from riders going around or 2) get off your bike and delicately walk around. I promise you won't lose an ounce of fitness and you'll set a good example for your buddies. If anything, you'll earn karma points.
Trail Leaders, please post in the comments as conditions change.