I would say that it doesn't signify is that okay? so much as tell me more.

It also suggests empathy in addition to agreement. To me, it seems roughly equivalent to I totally agree with you, you know? In addition to sharing the opinion, it also subtly connotes that both parties arrived at the same conclusion, possibly in the same way.

Slightly OT, but it reminds me of the similar (but not identical) Canadian English phrase:

That's cool, eh?

It's a sentence softener... and a way to get general acceptance from the person you are speaking with.

Anyway, best described with an example:

girl 1: That's girl's outfit is SO ugly!

girl 2: OMG, how does she even live with herself?

girl 1: I know, right? She is SOooo uncool.

"I know" can be a strong statement, so adding the question "right?" is a way of getting general acceptance from the other person.

It seems to me that "right?" is a way of creating a bond between the two people talking. I usually hear it in the context of sharing some fact that you wouldn't tell someone to his/her face, e.g. "How does she even live with herself?" from Atomix's example. It's like saying, "Right? We share this idea and therefore we are similar and should be friends."

Emphatic agreement in a youthful, maybe feminine register.

It is a friendly assertion that the speaker has already had the same idea–as in, "yeah, I know"

According to Urbandictionary its,

An affirmation that you agree with or can relate to the preceding
statement. It can be used whether the speaker actually knows or not,
but in the latter case it usually means that the speaker can attribute
the preceding statement to themselves as well.

Some examples include,

Student: I couldn't pay attention to the lecture because of that ball of sweat hanging from the professor's nose.

Classmate: I know, right?

Carrie: I can't belive Alvin cut her hair like that!

Lisha: I know, right?

I know; right?

I think we can break the slangy idiom “I know, right?” down into two parts:

• 
  I know (that). ― I get what you mean. I've had the same thought. I've been aware of it as well.


• (Yeah, it is,) right? ― It is really so, right?

By the literal interpretation of the sentence, it could grammatically mean to ask whether the speaker, “I,” knows something or not; albeit not making much sense in the context. So it would be reasonable to consider it to be a combined contraction of the two sentences above showing the consentience between the speaker and the lister(s).

And “I know; right?” might be more preferable punctuation in this sense.

About 20 years ago there was an idiom that I observed that people used for agreement with a statement. "I know that's right!" with particular emphasis on "that's". I wonder if what you're asking about is a combination of a shortening of that with the fairly recent tendency among some to make every statement sound like a question (with a rising pitch) as if the speaker lacks confidence in what they're saying.

I visited because I'm using the phrase myself and have been doing so for about a year. I moved to NYC about three years ago, and I'm wondering where this particular phrase came from in my vocabulary. I really like it. For me, it does indicate collusion ("I know") and a request that the collusion be validated ("right?"). However, I believe the expectation of validation is already implied because the rising action of "right?" is generally very subtle and casual on my part. I'm expecting the person to already validate my attempt at collusion.

The phrase also seems chummy, a way of saying in shorthand, "I agree with you completely and therefore we are kindred spirits." I usually say it almost as a bestowal of praise upon the other person, as if to say, "What a wonderful person I have found who validates my existence so completely with their similar thoughts on things that I had not expected people to think similarly about. What a pleasant surprise!" Anyway, those are just my thoughts. It probably did become abundant because of television, but it definitely seeems like something that would arise in either the Midwest, Deep South, or California (all of which I have ties to) because of a need in those areas to validate community over individuality and that I would feel more compelled to use in the Northeast, where I might feel isolated and want to frequently make references to ways that I might bond or fit in with others in my community.

I think it doesn't mean anything, it's just become an expression, because young people seem to enjoy talking in "code".

I would perseive it as

"I know it, did you hear me?"

"Stop telling this to me, it unnecessarily bothers me because I already know it"