Depends what you mean by "grip." Negative camber turns the wheels in towards the top of the vehicle. As you can imagine, this distorts the contact patch, resulting in it narrowing slightly at rest. So, generally, in a straight line you'll see decreased grip by running more camber. That's the trade-off. The benefit is that you'll get increased grip on the corners. As the car compresses onto the outside tires into a turn, that tire levels out. What used to hit the ground at an angle is now square, providing better grip.
The above is a simplification, but that's the general idea.