Buy a bar of Naptha soap in laundry area (Walmart carries). Wet areas n rub bar across area several times n throw in washer - wash normal. Can mix with other clothes n add
detergent as normal
That is a tuff one. It is good to start with fresh shirts, each time you wash spray with a stain remover so that body fluids don't build up. you may also try: rit dye color remover. directions are simple.
Another option for removing perspiration stains is to make a damp paste with Borox washing powder. Rub the stain portions of the shirt together vigorously, and rinse well. Repeat if necessary, before running through a washing machine cycle with hot water.
If the stains on the fabric have been laundered and heat dried, then unfortunately, they may be already set, especially if there is any polyester in the fibre content of the shirts. But it is worth a go to try to clean them.