I have been searching the Internet for an answer to this question for while now, but to no avail.
If I walk into a room and there's a person there among my friends that I don't know, in English could say something like "Oh, there's a new face" and go and greet him or her.
In Japanese, should I use "ああ 新しい顔があります！" or "ああ 新しい顔がいます！"?
The face is attached to a living person, but the face itself is not a living thing in the same regard.
I would be most grateful for an explanation.
In Japanese, one does not use "いる" to describe things that happen to be attached to living persons.
You would use "ある" for body-parts, prosthetics, (internal) organs, clothing, hair, accessories, acne and makeup (although there are more appropriate words depending on what you are saying). The list goes on.
The exception would be if whatever item you were describing needed to be personified. Say you had a really advanced prosthetic, which had an advanced A.I. that could speak to people, and perhaps convince them that it was "alive". Or, if we're really getting creative, say you had a sentient zit. These kinds of rare cases might call for using "いる".
If I am placed in the situation you describe, I say "初めて会う人がいる or 初対面の人がいる". And I think we don't say "ああ 新しい顔があります！" or "ああ 新しい顔がいます！". It's the direct translation of an English saying.
In addition, If the situation is in school, we call them 転入生 and 転校生. If the situation is in a company, we call them 新人.
