I have two methods for roasts on the grill. They usually start with a rub of sage/thyme/rosemary, crushed garlic, and dijon mustard applied to the roast (after salt and pepper). The divergence depends on what wood I have in the garage.
If I have a piece of cedar I cut a piece the size of the roast, soak it in water for several hours, build up some coals on one side of my grill, and then place the roast on the board on the far side. Roast until done.
If I don't have a cedar board I do the same thing, but place the roast on a folded up piece of aluminum foil.
I like mine meat medium rare so I tend to go for around 130-135F before pulling it off. It helps if the roast is room temp before you slap it on.