I applied in-person. The process took 3 days. I interviewed at Greenpeace (San Diego, CA) in Jan 2015
Interview
Find the ad on craigslist and call the phone number. They will give you a time to come to Greenpeace office for your first interview. You will be interviewed with a group of strangers, and they will basically just assess how you interact with people and how pumped you are for the environment.
I went through one interview via the manager in the main office. It was a few questions and overall very easy. I was first approached by another canvasser on the street and pulled in for an interview.
One interview and then a three day period to get 3 new members, team was helpful in supporting you. Some of it is luck of the draw though, I moved to Boston before securing this and it could have very easily not worked out