The Ghost Club, based in London, UK, is the oldest paranormal investigative group in the world. It grew from a group of fellows at Trinity College, and was officially established in 1862. Members have included author Charles Dickens, poet Siegfried Sassoon, Harry Price, race car driver Donald Campbell, actor Peter Cushing, Tasmanian governor Peter Underwood, paranormal investigator Maurice Grosse, Irish diplomat Sir Shane Leslie, and author Eric Maple.
The group conducts about 10 investigations per year around the United Kingdom, and has a rather impressive quarterly newsletter which covers investigations, personal accounts of the paranormal, and interesting developments in the study of ghosts.
It is worth noting that membership is open to people who live overseas too. Membership gives you the chance to attend meetings and investigations, and to meet some of the most prominent researchers in the world. You will also have access to the most recent newsletters, which are extremely well done and very interesting.
See their investigation reports at no charge (scroll to Past Investigations at the bottom-left of the page).
Many of the past newsletters (going back to the late 1990s) are available for download at https://www.ghostclub.org.uk/newsletters.htm
If you are interested in becoming a member, go to https://www.ghostclub.org.uk/application_online.html. The annual dues as of this writing are £25 per year.