Can I just use plain old brown sugar? And if it’s supposed to be a body scrub, why are the coffee grounds so fine in texture. Would it not make more sense to use instant coffee?

Instant coffee will just disintegrate or "melt". Coffee grounds retain their texture - do not "melt". You could use just plain old sugar or salt. Another good scrub: put a handful of oatmeal in a blender until very fine. Makes a great soothing face mask, especially for sunburned skin. Also, be careful your scrub is not too abrasive…you just want to remove dead skin, not irritate the live skin!