What I have done when dealing with large groups in the past is to use the following system:
1. Calculate the group's total load from their adventuring equipment plus travel supplies.
2. Assume that the group is evenly distributing load in a smart and efficient way and divide total load by number of party members to work out average encumbrance. Use this to determine the group's wilderness movement rate.
3. Use the average value to know how much weight they can pick up or shed before changing movement rate.
This does create the possibility that a particularly laden fighter (say, in Plate Armor + Shield + Spear + Longbow) may have an average encumbrance that's lower than his actual total encumbrance. In most cases you can assume he's not wearing/carrying his cuirass all the time, sometimes riding on his horse, etc. If they have a surprise wilderness encounter, you can worry about it then, otherwise it won't come up.
EXAMPLE: A party of 10 characters goes on a wilderness expedition. Their adventuring gear works out to 60 stone total, for an average of 6 stone per PC. They have 1 weeks' food and drink each (7 stone each), bringing them up to 13 stone per PC. They will move at 30' per turn, or 6 miles per day. After 3 days, they've consumed 3 stone each, putting them at 10 stone each, increasing speed to 60' per turn, 12 miles per day.
If mounts are used:
1. Calculate the group's total load from their adventuring equipment plus travel supplies, including the mounts' supplies. Mounts use 4 stone per day in supplies in the desert.
2. Load up the mounts to half encumbrance first
3. Then divide the remaining weight among the adventurers up to 10 stone
4. Then divide the remaining weight among the mounts up to max encumbrance.
5. Then divide the remaining weight among the party.
6. The party's movement rate is the lesser of the mount's or the party's. It will usually be 60'.
EXAMPLE: The party of 10, above, brings 5 mules with them. Mounts use 4 stone of food and drink per day. 1 week of food and drink for adventurers and mount will weigh (1st/day x 7 days x 10 adventurers)+(4st/day x 7 days x 5 mules) = 70 + 140 = 210 stone. Add 60 stone for adventuring gear to get 270 stone.
Each mule can carry 20 stone or 40 stone loaded. So the 5 mules are initially assigned (20 x 5) 100 stone at light encumbrance. That leaves 170 stone. Therefore each adventurer is assigned to carry 10 stone. That leaves (170 - 10x10) 70 stone. The 5 mules each carry (70/5=14) 14 more stone, for a total of 34 each.
The party's movement rate is 60' per turn, or 12 miles per day, e.g. twice as fast as the party that didn't bring mules.