Two questions

I was just wondering if someone could answer two questions for me?

First, is there an informal greeting? The equivalent of "Hi" or "Hello" in English as opposed to Ohayoo ("Good morning"), Konnichiwa ("Good afternoon") and Konbanwa ("Good evening") when meeting someone in person?

Second, how would you say "Have you met Denise?" in Japanese?

You can say "ossu", but only boys use this and you can't use this to your older or superior.
Also you can say "domo" like hi, but it's not so frendly.

Actually you can use konnichiwa whenever. I consider that Konnichiwa is Hello in English. Of course you can say hello is Good after noon in English though.
And you should change your greetings to suit the occasion.

Have you met Denise?
Denise ni atta koto ga arimasu ka?

Thank you Ritzy