Some Organizations involving amateur - professional astronomy

First: an organization relevent to all amateurs (and professionals): International Dark-Sky Organization. This group spends its energy helping protect dark skies, and educating the public, government, and utilities on the efficient use of lighting. Most outdoor lighting is horrendously inefficient, sending as much light up into the sky as down on the ground where it is (presumably) wanted.

• Center for Backyard AstrophysicsStudies cataclysmic variables in people's back yards.
• American Association of Variable Star Observers
• International Amateur-Professional Photoelectric Photometry This group has been arranging amateur participation in astronomical projects via newsletter and local groups for some time, but doesn't have much of a web presence. International Amateur-Professional Photoelectric Photometry, Western Wing A regional wing of the IAPPP that is trying to develop a web presence.
• International Occultation Timing Association studies Lunar and Asteroid occultations.
• European Asteroidal Occultation Network
• Collaborative Asteroid Lightcurve Link Request or provide asteroid lightcurves.