How about some more background on how they're managed?

Imbalanced diet, or bumble foot caused by damp muddy conditions or something in the bedding or run area such as glass or old nails are a few guesses that come to mind without knowing much about how they're kept.

Hi Patrick,
Guineas have one of the horse stalls (12x12) that opens to an outside run (8x12). Stall floor has stall mats over stone dust, floor is covered with 4" (+/-) shavings & hay.
They are fed BlueSeal Turkey Grower & white millet.
Outside run is dirt, no mud.

Although bumblefoot may be contageous under certain conditions, the only thing that you're doing that isn't commonly done by most other bird keepers is using the stall mats. Even though rubber, it's possible that it's too solid, and they injure the foot pads enough on flying down from perches to allow the introduction of the bacteria which commonly causes bumblefoot. They also may be slipping in such a way on impact to injur or stress muscles and/or tendons. Either way, I would try bedding the stall with a good layer of shavings for a few months and see if that doesn't help.

Thanks Patrick, I'll add some more shavings to the stall. When I checked the lame birds today they seemed to be doing somewhat better... they were up and walking with only a slight hint of a limp. We did have a rodent problem that has since been resolved, perhaps the hens were being tormented at night by the intruders.

How are they limping? One legged? down on hocks?

Maybe botulism? Do you keep their water bowls clean? It could be so many things.

So many things, yes. Botulism, no. Rodents spooking them at night, causing injuries, or even chewing on feet or toes is a possibility.