For dandruff, you might use shampoos containing ZPTO and Ketoconazole which can be had as OTC product. Dandruff is a feature of seborrhic dermatitis that can affect groins too. However it is better to consult your dermatologist as above measures will surely give you relief but in the absence of proper diagnosis, taking self medication for too long might alter the original picture.Such symptoms could be due to another skin condition as well which is called Psoriasis and which needs different medications. However the suggested shampoo will provide you some relief for now