WildStar is getting the free-to-play treatment sometime this fall. To help ensure that transition to the new model is seamless and that all features work properly, they've commissioned a closed-beta test.
Any active account can participate in the WildStar closed-beta.
Last May it was revealed that in order to stay relevant and continue growing, Wildstar was shifting to a F2P model. They wish to keep their whimsical cartoon like dream alive, and this seems a necessary step to facilitate that.
This closed-test is a way for NCSoft to ensure that most (if not all) of their ducks (or gribbons) are in line before going live. They want to ensure that everything that's supposed to be open and accessible to free players works as intended. This is also a fairly important feature update as well, with changes to the in-game currency, modifications to movement, dungeon improvements and a few relevant and much needed quality of life improvements.
Just remember that if you participate, this is indeed a beta test and issues are expected, with you the tester helping to squash those bugs. If you plan on partaking, please do the sensible thing and report such issues to help with the development process instead of simply complaining about them. The more players they can get into the test environment, the better they can test the servers and features.
You can go here to sign-up for the closed-beta. Currently, WildStar has around 200,000 subscribers.