It is a game which involves a group of people, preferably the more agile, to go to a childrens playground (the more complex the better) and to have a contest as to see who is it and the others are to hide around upon the playground works while the person who is it tries to find these others while the it person has closed eyes or is blind-folded. The others can move to other areas of the playground but must take great care when moving or especially on the ground. If the person who is it calls out GROUNDERS while someone is traversing the ground to another area that person becomes it. Another way to pass the it status is to tag someone.
The game usually becomes more fun if the participants are intoxicated on alcohol or marijuana or other things. It is also fun if a member is overweight or typically scared of jumping around or having ones eyes closed on complex surfaces!
Grounders is a game that I only know of existing in Vancouver, British Columbia but may exist elsewhere.
* These videos are coming directly from Youtube, they may or may not be most relevant to the word "Grounders"