"One of them was/were you"
If I am talking to somebody about a certain group of people in the third person, and then want to refer to the person I am talking with as one of those people, which do I say?
One of them were you
One of them was you.
The verb must agree with the subject 'one', therefore it's 'one of them was you'.
"One of them" is a noun phrase referring to a single person. Therefore all its verbs are always singular.
This is correct:
One of them was you.
This is not:
One of them were you.