The “Beverly Hills 90210” reunion continues with — the big buzz is true — Shannen Doherty (Brenda Walsh) joining former castmates Tori Spelling (Donna Martin), Jennie Garth (Kelly Taylor) and Joe E. Tata (Nat) on the new “90210,” which premieres Sept. 2 on the CW. The tempestuous Doherty, whose last noteworthy role was Pru on “Charmed” a decade ago, left ‘90210’ after the fourth season with the explanation that Brenda went to London to study acting. Now she returns to her old high school (for “multiple episodes”) as a successful actress who has become guest director of the school’s musical production. Spelling will return as the owner of an upscale boutique, Garth as a guidance counselor and and Tata as the owner of The Peach Pit.