These are tricky problems. I understand it to be that positive is the addition of something and negative is the removal of something. If is kid is throwing his toy at their sibling a positive punishment would be yelling at the kid, and a negative punishment would be taking the toy away. In the case of grounding, unless the question specifically states that the grounding included the removal of video games, then I would probably consider it to be a positive punishment because there was something new introduced: the "grounding".
** I researched a bit more and I found sources saying it is positive and sources saying it is negative punishment...so, I guess I'm a bit confused as well..