Rash on groin area

Probably a heat rash develops in the groin area.
Keep the area clean and dry, try some baby powder and keep something soft on in while wearing pants or maybe keep a bandage on it.

The sore on back of your neck can be Sebaceous gland cyst that arise around the sites of hair follicles and thus will often be found where hair grows, most commonly on the scalp, neck, back, face, and arms.

The treatment of a symptomatic sebaceous cyst is to either leave it alone or surgically excise it, after which it should not recur. However if has become infected a course of antibiotics is needed to eradicate the microbes causing the infection before considering excision. Most importantly if this turns out to be your diagnosis then you need not worry about it. My advice would be to see a doctor in person to examine it and take samples if pus is being produced from the cyst.