I was told (by a couple of teachers) that the correct usage is, "It's they." I think it should be, "It's them." Can you confirm and tell me why?
A linking verb (in this case is) does not connect the subject to an object. This type of construction should be reversible. It is they. They are it. I am Mike. Mike is I.
That being said, EVERY English speaker I've ever had a conversation with says "It's them," or something like that. It is just natural for us to use objective forms of pronouns when they appear in places in sentences where objects normally occur.