The way they is used in English as a collective and a singular noun, either of those verb tenses would be technically correct, but I'd say that it's more common to use "are" in that example than "is."
Never doubt that a small group of thoughtful, committed citizens can change the world. Indeed, it is the only thing that ever has. - Margaret Mead
I've thought a lot about this for years. People always say "are they" and "they're" and "they visit" (when singular would be "they visits). The exception to this is that the word "themself" has slowly been being popularized.
Personally, I prefer people to refer to me as they/them singular (is they, they's, they visits) because even though it sounds weird I do it rebelliously. Like, I don't want to use neopronouns, but I still want to be annoying to people who aren't eager to accept pronouns. I'm definitely the exception, though.