Answer:
Improve
As far as I know, the only way to actually join the green police is to be a currently serving police officer in the greater New York metropolitan area.
However, if you want to get involved with social change for the environment, there are plenty of local and national groups you can join to make a difference such as:
Green Peace: (Eco watchdog with local chapters)
http://www.greenpeace.org/usa
Slow Food Movement: (Focus on sustainable, local, organic consumption)
http://www.slowfood.com
I don't think the green police is an open organization but I could be wrong.
But there is a call for Green Police volunteers for the Glastonbury Festival.
Other note: The green police is not really about authority or being an official member, it's about awareness. Being concious of the impact that we all have on the environment. You don't have to join a group or get a badge for that.