MidnightSensi2,
In my opinion you can expect to obtain clean, essentially noise-free images up to ISO 1600. This ties in nicely with dpreview's review which suggests ISO 3200. While it is clearly impossible to know at this time just how dimly lit the venue will be, it is not unreasonable to expect your images to be made at around ISO 800 to ISO 3200, so good news there.
1. Auto-ISO used in conjunction with Aperture Priority at, likely, maximum aperture is the way to go. Consequently, use your fastest lens(es).
2. Where possible use your surroundings like a guard rail or balcony to hold the camera as steady as possible when shooting.
3. Use Continuous autofocus because your subject (people) will rarely be stationary (except perhaps at the end of the evening
). Consequently, take a spare battery if you have one.
4. If your lens has Image Stabilisation switch it off, as the mechanism normally requires approximately 1 second to stabilise before a sharp image can be made.
5. Use a flash if you have one, where using it will have a positive effect and, if it is not a private party, if you are permitted (check with the venue for indoor photography - outdoor photography is not likely to be a problem).
6. Use centre weighted or spot metering in preference to matrix metering, or you may end up only with silhouettes of people in your images.
7. Shoot RAW if possible for added security in potentially 'rescuing' images later.
You will need to experiment with shutter speed (Shutter Priority) as the correct setting to achieve the degree of blur you desire will be a function of focal length and subject speed. The wider the angle of view, the slower the shutter speed required. The faster the subject movement, the faster the shutter speed required. As an
approximate range, you are likely to be looking at 1/4s to 2s or 3s, possibly 5s. If you leave the shutter open long enough (bulb setting) you can even make everyone disappear.
If your employer is unwilling to hire a Professional photographer then they should be realistic in their expectation. It's a party, so after a short space of time everyone will be, um, 'refreshed' and increasingly loosen up.
Virtually everyone will be keen to have their photo taken individually or in a group...especially the ladies.
Relax and enjoy your role - you will be the envy of all the males.
Hope this helps.