When they are not closed due to rain (way too often recently), I will typically ride USNWC during the week nights. I have arrived there as late as 8:30pm and still rode.
Regarding the guard being on duty, every time I have left, even as late as 11pm, there has been a guard there. I have seen other postings indicating that you can ride till 11pm, but I have not seen it documented anywhere on their website.
The later it gets, the fewer riders that will be out, though. It can seem strange to be in a parking lot setup for hundreds of cars when there are no other cars in the parking lots, except for the USNWC's Subarus. I have seen a few cleaning staff there that late, so the place is not totally empty
As far as group rides, look for the thread 'Trailblazer Tuesday Night Social' -- it is a Tues night ride that changes locations. I have seen another thread where some people ride at USNWC on Thursday nights.
There are 2 other regular night rides in CLT -- Monday nights at Fisher Farm (northside) and Wednesday Nights at Beatty (southside) -- there are threads for each of these rides as well.
Good luck with the ride and may the weather be with you!!