The GDB gang does a Tuesday/Thursday night ride going down the White Rock Trail from Forest Lane, around the lake, and back up, for ~21 miles.  There are typically anywhere from 6 to 20 people involved, and it usually breaks up into different speed groups, with certain regrouping points.
 
I rode it last night, and just wanted to post some general thoughts:
 
1)  Any group ride going down the WRTrail will raise a LOT of dust, especially if you're behind a buncha DFs with 2" knobby tires...   Keep this in mind, and wear 1) clear glasses of some sort, and 2) have a scarf/dustmask/balaclava or something to filter your breathing air.  Or just stay in the front!   The dust raised reminds me of the Lime Plant at Cleburne... but worse!
 
2)  The Trail is colder than the parking lot.  It's typically 5 degrees or more colder along the trail, then warms up again at the lake.  However, when you're going back up the trail, it's 90 minutes colder than it was when you started!  So dress accordingly!    Last night it was ~60 at the parking lot, but easily approaching the 40's on the trail on the way back...
 
3)  The more light, the better -- although when riding in a group, multiple lights help.  I broke down and bought an HID this year (for TTTT, actually), because my night vision isn't the greatest.
 
4)  I prefer wearing my light on my helmet for the WRTrail, as opposed to on the handlebars --with  the twists and turns along the trail, it's good to be able to shift the beam around to look around corners.  This is ESPECIALLY important due to the occasionally walker(s) one encounters who are wearing NO reflective gear whatsoever!!
 
5)  When night riding in a group, single file, you have NO visual idea who is directly behind you, or who is or isn't keeping up -- all you can see in the mirror is headlights...  I inadvertently left Bill Sutton behind last night, who wound up having a flat at Abrams.   He apparently fixed it and turned around.  Sorry, Bill!  
 
6)  The majority of the GDB gang tends to ride at 18+, although we're trying to stress caution on the Trail.  However, slower groups form up -- but you have to be proactive about it, don't be afraid to ask or suggest it at the start.  
 
7)  Even a casual-paced (14-15mph) ride with a couple stops takes no more than two hours.