I'm not sure a "420 crowd", or "stoners" would be very unwelcoming to outsiders. Any group that you might want to "infiltrate" will probably easily pick up on such an attempt.
When you approach a situation thinking of any particular group as "them", you begin to generalize the people in that group. Even if the generalizations are resting at the back of your mind, they would likely come out in your actions as verbal cues or involuntarily body language if you aren't some kind of actor/actress.
Of course not everybody is equally capable of picking up on these cues, but my case in point is that it's probably best to remain natural and not break it down into steps. I believe it will all come down to your deductive skills and self control.
Youth is not enough, love is not enough, and success is not enough... And, if we could achieve it, enough would not be enough.