Students who already have taken refuge are welcome to renew their vows.
At the end of the ceremony, it is traditional to offer two katas (silk offering scarfs, available in the bookstore) - one to the Buddha statue on the altar, and one to the teacher giving the vows, who returns it by placing it over your head as a blessing. While we often have spare ones to lend, in this case it is best to buy them so you are actually making an offering of your own. It is also auspicious to give the teacher a gift, typically money (any amount) in an envelope, in appreciation for the incredible gift he has just given you with this opportunity. The center will also send you a PDF of the FPMT booklet on refuge and how to practice it daily.
Learn more about vows from this article in FPMT's Mandala Magazine
Masks are recommended/requested.