“Invites will be sent to a large group of new PTS testers, but if you participated in the ESO: Morrowind PTS beta last year, you’ll be able to log into the Summerset PTS from the ESO launcher yourself, no invite required! Just don’t forget to enable ‘Show Public Test Environment’ from your launcher settings. Note that as always, anybody can install the PTS client, log in, and test the base game changes, but they won’t be able visit Summerset or experience any of the new Chapter’s features unless granted access.”
Do note that the main quest and story are under a strict NDA (agreement, watermarked screenies, the works) that lifts May 1st.
MOP’s Andrew and Eliot got hands-on with the chapter at GDC and PAX respectively, both of them finding it to be basically exactly what Elder Scrolls Online fans want and expect out of a big content dump.