lens - Nightclubs photography, setup to capture the real mood and atmosphere?

I was hired to photograph at a nightclub on Friday. I've never done this before, and I do not have any specification for the moment.

I'd like to be able to capture the atmosphere at the right moment, play with the lighting etc.

I assume I'll also have to take some shots of people gathering and have their face/costumes on photo to show them off for their friends on Facebook and the likes.

I have a default kit: Nikon D5100 with AF-S DX VR 18 - 55 mm f/3.5 lens

Do I really need to invest in some TTL Flash, and another lens?

I've read different advice from people, some prefer to use a flash, some rather not use them. What's the best choice in order to stay true to the atmosphere and mood?

Is using 800 ISO best, or should I go higher?

Would it be preferable to rent a lens for the night and go for a f/2.8?