Disclaimer: I've searched for the answer of this question quite extensively online but I never got anything 100% reliable. The best I got was the answer a friend gave me because his friends has done this (open Dark Grounds during Rapid Respawn). So, take what I will say with a grain of salt. It has not been proved nor verified, but I doubt you will find such thing online.
To open Dark Grounds the server has to collectively complete the task Mend What's Broken 80 times throughout as many server saves are need.
I have no information about how long it stays open (if a single or multiple server saves) and if there's anything that can be done to keep it open or to close it.