There are quite a few ground points throughout the car.
I found this to be the case (as with all cars) when I stripped the interior of my fortwo to apply sound insulation and install the stereo. If you look at the photos in my Gallery, you can plainly see a few of them-the brown wires terminating at a nut. The easiest ones to find on your own are on either side of the battery, and behind the access panels in the cargo area. You'll need to remove the seats and carpeting to get to those under/behind the driver's seat and console, and there are a few way up under the dash. If you change your horns/springs/oil/exhaust you can find a bunch more.
They're all secured with the same size nut found on the sides of the battery, and they're on really tight. I wouldn't worry about it unless you disconnect one-be sure to torque it back down.
My experience has been that plug-in connectors have more of a risk of being a problem. My car lost its A/C a mile away from the dealer on the first ride home due to a connector that wasn't locked together properly at the factory. It took the dealer three hours to find it because it looked fine as they looked through the car. They said they had seen a couple of cars where things failed soon after the car was new due to improperly-seated connectors in the harness. They also had to replace an entire wiring harness due to mice chewing on a couple of wires. (Repair sections and individual plug connectors are not available, so the entire harness had to be replaced. So if you disconnect things [like if you're a noobie trying to remove the seats], don't break the connectors!!)