Many sources state it is set in the same universe as Space: 1999 (1975), as it was originally meant to be the series's second season, and later a spin-off series titled "UFO: 1999", mainly focusing on space warfare, but later evolved into a separate series.
Yes, there are a few of them. One of the most famous is the Italian Section Of SHADO (ISOSHADO), based in Milan, Italy (the series had a great following in that country). ISOSHADO's website is very, very well-done, with huge bilingual sections about episodes, films, merchandising, events, and even photos of stars from the series coming to see the club's headquarters.
The following web page is an excellent on-line FAQ:
Follows the missions of the Supreme Headquarters Alien Defence Organization (SHADO) a secret organization which defends Earth from extra-terrestrial threats by a dying alien race who need to harvest human organs to survive.
Who are the characters in UFO?
Capt. Steve Minto, Carol Roper, Doug Jackson, Dr. Frazer, Dr. Segal, John Rutland, Keith Ford, Lt. Andy Conroy, Mary Rutland, Miss Ealand, and others
