James, I am researching this very issue for my replacement rope. Im replacing my HEAVY and unnecessarily strong BluewaterII+Plus 7/16"x150' with a 7.7mm ice twin rope.
I learned a simple rappel technique I'll explain below, but first let me repeat what I recently learned in "The Mountaineering Handbook"[Connally]; Security is more critical than Strength when rapelling. The strength of modern cordage is more than enough. The most force you could put on a rope during a rappel is about 1.5KN for an 75kg person; so any kernmantle rope above 5mm is more than enough. Even 8mm accessory cord is about 12KN. So, you could rappel a doubled 7mm for a feeling of security, or single 8mm for example. Most people I rappeled with in the past learned on a double, and are comfortable with only that; what about the members of your group?
Technique:
Im assuming you want to minimize weight and number of items carried, and maybe cost. You could use one of the techniques the US Army teaches (I learned in Air Assault School years ago) for simple group rappels. With this system you won't need any ATCs, friction devices, or single use harnesses. All you need is one rope, one 4 m rope "swiss-seat" and locking carabiner per person, and whatever you need for your anchor; let's assume one sling and one rap ring. Plan on abandoning your anchor.
Assume a single-line rappel.. Each person makes a swiss-seat with 4m of 8mm rope, ties in with a Munter or Super(double)Munter if the rope is thin or wet. First guy down uses an autoblock knot to self-belay (requires more equipment), or just goes on down without one. Each guy after the first gets a fireman's belay from the previous one down. Last guy down pulls the retreival cord (maybe 3mm?), thus abandoning the sling and rap-ring in place.
Rope:
For a 100 foot rappel you might need 40m or so, including anchor and belay use (and that knot at the end of the rope!). So, you could buy the Beal Rando 8mm x 60m at REI for $109, cut 5 sections of that rope, at 4m each, for each member of your party (charge them $2 for their new "harness", and I assumed 5 guys), and you would still have a 40m section left over for your rappel. Total weight of the rope and 5 swiss-seats: 1.9kg!
For future hikes, that 40m section would be a useful length (not too short, not too long, not too heavy) to carry as a contingency to negotiate obstacles on YDS Class 3 or 4 hikes. Carry the 3mm x 40m retreival cord (REI:$19.80,) and a simple anchor, and suggest each member of future hikes carry at least one locking biner and a 4m harness; very light.
What I chose to replace my old rope is the Beal 7.7 Ice Twin "Can be used as a single rope on abseils". It is long enough to double up on any rappel I want to do, and if the rappel you mentioned is a little less than 100', you could double it there too? Its only $67.50 for a 60m rope! That's a smokin' deal.
http://www.backcountrygear.com/catalog/climbdetail.cfm/BE112
I do not know how much experience you have; probably more than me, so forgive that explanation if it was all obvious to you.
I found the book I mentioned an _Excellent_ text for the simple 5.1 climbing and non-vertical rappelling I do. I highly recommend it. (And 'Freedom of the Hills' of course)
References:
How to tie a swiss seat:
http://home.arcor.de/varn/rappel/swiss_seat.htm
I use a 4m rope and put another overhand in step 6; seems more comfortable for me.
Army TC 21-24; Rappelling
http://www.ajrotc.net/pubs.html

