I have the book but I haven't read it yet. I will say though, that I think seeing the mini-series first may actually enhance your reading experience. The TV series shows just how delightful and humorous some of the antiquated turns of phrase can be. I'm not sure I would have picked up on that on my own.
Look forward to reading your thoughts on the book and the adaptation. :) They're both a delight as Thomas mentioned. A few things: the plot is linear in the adaptation while in the book it jumps around (it was originally written as a serial for Dickens' Household Works). They also incorporated two novella's, Mr. Harrison's Confessions and My Lady Ludlow, plus one of Gaskell's essays: The Last Generation in England. But the changes don't affect the enjoyment of watching the program, if anything it makes it better. I have some resources (ie. publishing history, annotations, etc.) on my blog, if it would be helpful to your group read: Cranford