Past / Present

Dyske,

I'm not sure which is correct, gramatically, but the common usage would be "who I was". If I were you I would just say "only a few of them knew me."

It seems to me that nobody at the lecture yesterday could have known who you are, because perhaps the experience of not being known changed you and today you aren't the same person they saw. By this logic, you could say that it would be correct to say "nobody knew who I was" because you are also referring to yourself in the past tense, as the you of yesterday is not the same as the you of today.

"Who I was" is the correct way to phrase that. The tense within a sentence should agree the whole way through. "At the lecture yesterday, only a few of them knew who I am" sounds like they can divine the future, as in only a few of them knew then who you are now. If you wanted to state that only a few knew you 10 years ago, you would have to state that. Even then you should explicitly state that only a few of them knew who you were 10 years before the lecture (as opposed to 10 years ago which tends to suggest 10 years from now which could be a problem if the lecture was 20 years ago).

both are grammatically possible.

if the information being reported is still true (at the moment of reporting), then it is acceptable to say "...who i am". this assumes that the nuance of "who you are" hasn't changed, as the logic of orig alludes too below.

the use of "...who i was" is completely acceptable too, and tends to be viewed as more formal.

as dariensan mentions you do need to be careful of time references in your reported statement. there are rules that can easily be looked up in any grammar reference book.

Blend, if we could so easily refer to "any grammar reference book," there wouldn't be any need for this site. Or any fun either. :P

I especially like it that many speakers, of different dialects, can share views here in a living-language sort of way. Don't you?

Both are grammatical, but it's more natural to say "...who I was".