The easiest way to make a coffee rub is to add 1 tsp of dry coffee for every 1 cup of your typical rub recipe. However, this works only if your regular rub flavor is one that could be accented with the coffee flavor.
A quick online search for coffee rub recipes will lead you to a plethora of various rub recipes that feature coffee grounds. Your best bet for deciding is to go with a recipe that is similar to one that you know you already enjoy, then branch out and experiment from there if you decide you enjoy the coffee rubs.