I can vouch for MPP. They have been around for almost 20 years now and are by far the largest pro marijuana legalization lobbying group in America. I've been a donating member with them for about 17 years now and am constantly amazed with the huge role they have had in getting medical marijuana legalized in 18 states in america during that time period and now recreationally legal in 2 states with about 9-10 other states in position to do the same in the next 1-4 years. Obviously they arent the sole reason this trend has been happening but a lot of these state laws that have passed MPP had a large role in getting the law promoted if not outright helping draft the law themselves. In addition the numberous magazine, tv, radio, billboard ads and campaigns they have run across america.
They are legit and not only legit but very well ran and have actually delivered quite well on their goals especially considering how anti marijuana politicians and local/federal governments were (and still can be) just 20 years ago when MPP was first founded.