Acne on the buttocks (sometimes called "buttne") is often caused by folliculitis, which is inflammation of the hair follicles. This can be due to friction, sweat, or tight clothing. The hyperpigmentation (dark spots) they are seeing is a common after-effect of inflammation in the skin, especially in people with more melanin in their skin.
Next Steps
I'd be happy to assist further through an online consultation. This would allow me to get a clearer picture of the affected area and recommend appropriate steps.
Health Tips
Loose clothing: Wearing loose-fitting, breathable clothing can help reduce friction and sweating. Gentle cleansing: Using a mild, fragrance-free cleanser and avoiding harsh scrubbing can prevent further irritation. Exfoliation (with caution): Gentle exfoliation can help with hyperpigmentation, but it's important not to overdo it, as this can worsen the condition. Sun protection: Protecting the area from the sun is crucial, as sun exposure can darken hyperpigmentation.