As Jim said, it should already be grounded.
If your tank grounds are iffy (due to rust, corrosion, or other) then you can bring a wire down from one of the mounting bolts and ground it straight to the frame that way.
I need to rewire mine (The wire is bad -- and to add insult to injury -- bare in places! Half the time there's no gas or a full tank!) and I'm thinking about grounding it to the frame with a seperate wire just to be sure.
But, then again, I'm anal retentive about that sort of thing.
