Is using "it" with common names grammatically correct?
Who drink this Tea?. It's John.
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I disagree with the commenter who said "John does" is more idiomatic. If anything, that's a more "proper" response, but definitely not idiomatic and definitely not the way most English speakers (at least in the US) would respond.
If someone said, "Who's drinking this tea?", I would simply say "John." "It's John" also sounds fine, though. Or even "John is."
If someone said, "Who's tea is this?", I might say "John's" or "It's John's."
But, to answer your question, in general, you can use "it's" with names. The first example of this that I thought of is when you answer a phone: "Hello? Who's this?" "Hi, it's John."