I didn't know you were here

were is correct...

Thanks, Region.

The fact that my friend whom I didn't expect to be at the gathering IS present does not merit the use of 'are'. Is it not a general truth, just as in the case about the many devotees at the temple, which I stated in my earlier post?

I disagree with Phil, Tang. Yes we almost always say "were," but "are" is okay too; though when we do go that way we'd almost always say "you're here"

...especially appropriate if you'd been at the gathering for a long time or for instance if it's your house

"Were" is in keeping with the tense of "didn't," the past tense.

"Are" would be used in a present tense sentence such as, "I don't know if you are here or not."