Disclosure: I shot weddings last century. Won’t do them now at the age of 58 because of the sheer stress of getting things right on a very important day for the bride.
As others have stated, you
already have an important role at the wedding.
Sure, you could haul out a good-quality compact (with flash set a stop down from ambient) and snap away
at the reception for relaxed candids. I have done this with the likes of a Panasonic LX3 and more recently the GF1 with the brilliant 20/1.7. I underexpose both the flash and the ambient light using low-ish shutter speeds and wide apertures, taking many shots because some will
not work. Example from an art exhibition opening (GF1 20mm f2.8 1/60 iso 100):
But stepping out with a camera from your best man post to snap pix of the couple exchanging rings and having that veil-lifted kiss? Tacky.
There are photographs of any wedding that only an experienced photographer should be making. Why not approach some friends and pool resources to buy a photographer’s services as a wedding present? Traditionally this has been the responsibility of the bride’s family, but if they cannot afford it, another way
has to be found.
It should not be hard to find somebody who will attend the ceremony, and take some time for group shots after the ceremony, at a reasonable price. Then others, such as you, can fire away to make the folksy pix that will be used to fill out the album.