It's Fungus or yeast infection....common around skin folds like thighs and genitals.. avoid sweating, sharing clothes,towels and soap.. specific medicine required depends on the severity.. for detailed prescription do direct online consultation by sending photos.

Skin inflammation, usually in warm, moist areas, such as the groin or between skin folds is the condition u seem to mention. The other condition could be tinea cruris. It's a  Fungal infection in the skin of the genitals, inner thighs and buttocks. The condition most often occurs when people wear tight-fitting clothing that traps in moisture. Wear loose fitting clothes to begin with. Consult a Dermatologist for further evaluation.

Kindly consult online with relevant photographs for proper diagnosis and management