A number of us did this last year (at least 3 or 4 SpyderLovers for sure). If it works the same as last year, this is what happens....
Send in your Russell seat order during the group buy period and state that you are part of the group buy on the order form.
They send you a build date (anywhere from Oct. - Jan. or so, depending on where you fall into their queue).
You take your Spyder seat or a spare seat and send it to them, UPS ground, one week before your build date.
Your seat has the foam replaced and recovered to your specs. and is sent back to you.
You are without your Spyder seat for 2 - 3 weeks total for the rebuild and shipping time. If you get a spare Spyder seat to send in for the rebuild, you aren't without a seat at any time.
dltang and I got a spare seat from daddydarko, who had bought a Corbin seat. We sent that one in for her Russell rebuild. My seat build time was about one or two weeks after hers. Her Russell seat came back, we installed it, put her seat back in the box and shipped it to Russell for my seat rebuild, got my Russell seat about two weeks later, and sold my stock seat to another SpyderLover who was getting his seat rebuilt in the coming months.
As far as the group discount numbers go, they are looking for 50 people around the country to buy their seat upgrade. You don't have to be part of any special group or put together a group of 50 on your own to get the discount. Just put down that you want to be a part of the group buy and you're eligible for the discount. They need 50 or more orders to give the 25% discount. If they don't reach that number (they have exceeded 50 orders for at least the past few years), the group buy discount is somewhat less. I think it was 15% off if they had 30 - 49 orders and 10% off if they had less than 30 orders. Seems to me they reported 80 group buy orders last year so we got the full discount on a very nice seat.