Hi and welcome to the forums.
This Friday that is short notice
Are you going to be taking photos and selling them? If so how much are you chargins and how will you do this; print on the night or take orders? Will you be posting them in an online gallery for people to purchase?
Are you planning on shooting indoors or outdoors? If you are shooting indoors are you planning on hiring some portable studio flash and background as I doubt a single on camera flash (especially the SB400) will cut much dice.
Top Tips I would give is
1) Find out what the customer wants and shoot it
2) Be careful with your backgrounds (if outside use the widest aperture possible to throw the background out of focus, ideally you'd have an f2.8)
3) Be careful about the positioning of the people make sure you can see everyone's face
4) Dont shoot with a small focal length, otherwise you distort the people at edges of the picture and they will look ugly!!
I wish you the very best of luck - I think you need it.
P.S. If you are based in the North West and want help PM me with the details as I may be able to help you out for the evening. I have a secure online purchasing facility for parents (the ones with the money) to buy from.