I would ask the people there, as some horses are better left on their own for awhile as they get into better physical health.
I think merely spending time with them, with the rescue's permission, is a good direction to take. When they are fit enough to work with, then you should wait for the rescue to give you a task to work with them on.
Horses, as you probably know, think backwards from us as they are flight animals and we are predators. If you do something wrong, since these are rescue cases they aren't typically healthy in mind and may react in a surprising and dangerous manner. Better to be safe than sorry and ask for guidance by the rescue every step of the way.
"Strength is the ability to use a muscle without tension"