I absolutely love the rules for groups and fates but when they combine I get a little confused. I had a similar problem when running Apocalypse World with gangs healing, where there are very flavorful and interesting consequences for groups taking damage but not a similar hard and fast way to 'heal' them.
The simple way is that they heal the same way people do, just assume that if you're enough of a badass to have a group of warriors under your command over time you'll just replace casualties over time. I tended to just have them take the losses, perhaps shrinking in size in AW rules, and then just have them roleplay out the recruitment process but this quickly became boring and repetitive.
Maybe some sideline move, or just some simple healing ideas like normal fates.
(Or it's already been covered and I just missed it).