My Advice for wet weather planning...
This photographer will always have a umbrella with him! Notice it's transparent (to let the light through) and a ladies' style (deep, to keep you dry).
If the worst comes, it's always fun as a couple to have a couple of shots in the rain (but only if you're up for it)! The combination of the transparent brolly and backlighting with flash can make for some real wow-factor shots, freezing the rain with a fast shutter speed.
But, more seriously, my advice is that of course this isn't practical for formal group shots.
Do ensure that you discuss with your venue well in advance what their protocol for rain is - you need to know there is an indoor space designated that is large enough for you formal group wedding party photographs, that won't be otherwise dressed and occupied for the ceremony, wedding breakfast or reception.
Somewhere large enough, with ample natural light; perhaps the bottom of a grand staircase or similar, with large windows providing light.
Just a thought - forewarned is fore-armed - if you've planned, there's no risk of stress on the day!
And nobody wants to look like the idiot below...
Tom xx