Time for some musings from a DM who is still somewhat green in relation to 3rd Edition...
My group size is usually in the 4-8 range, lately 6-8. I've tried three different ways of forming challenges for a group larger than the 4 player party that all the CR/EL stuff is based on.
At first I kept organizing challenges for the average party level. This ended up creating a situation where the party would go through a lot of encounters before it started to wear them down. The inidividual encounter wasn't too challenging for them sometimes to the point of a rather boring series of encounters, and progression was slow at about a level every 6-8 weeks.
The second way I did it was to add up the party levels and only divide them by 4, designing the challenges based on this number as the party level. This created a noticeable jump up in difficulty, as the party was facing individuals that were too tough for them but they had to numbers to absorb the high frequency of players being knocked below 0 and still come out on top. This seemed acceptable for a while, but I noticed that my players were getting frustrated with the high difficulty level and the fact that at least 1 or 2 party members were knocked below 0 in almost every fight. The 2 clerics in the party were very busy with the Cure Minors for stabilizing unconscious party members while I used this method. Progression was increased to roughly 4 weeks/level, but so was the frequency of unconsciousness and/or fatality.
The way I do it now is to calculate the average party level, generate challenges based on that difficulty level, and then increase the number of enemies according to the proportion of players over 4 in the party during a session. So, for an arbitrary example to illustrate this, if my average party level is 10 a suitable encounter might be 4 Osyluths (CR 6, 4 of them is EL 10). If I have 6 players present, I just add an extra 50% to the creature numbers. In this case I'd use 6 Osyluths. For single monsters or numbers where the proportional increase doesn't work out to whole numbers I tend to make up the difference by increasing the hit points of the enemies but not the potency of their attacks. I've found that this keeps various abilities of enemies (damage, saving throws, hit chances, etc) in a range that PCs can handle individually, but the increased numbers and/or staying power of the enemies means the party still gets an appropriate challenge. Progression is still at about 4 weeks/level, but the players seem to be enjoying it a lot more because their abilities are more effective against the foes they are facing.