Pen Collectors of America (USA): One of the oldest and largest collector organizations, the PCA publishes The PENnant three times per year. Vintage pens remain the primary focus, with solidly researched original articles. Perhaps the greatest benefit of PCA membership, however, is one that most members pass by: the opportunity to download copies of old catalogs, advertisements, repair manuals, in-house journals, and other original literature from the Reference Library, an unparalleled storehouse of information.
Writing Equipment Society (UK): Fountain pens are only a part of this group's interests. The Journal of the WES is published three times per year. Meetings are held several times each year, mostly in or near London. One can now join online and pay membership dues by Paypal -- a real boon for overseas members.
Israel Pen Club: The organization still appears to be active, but no more website.
Accademia Italiana della Penna Stilografica (Italy): Once upon a time the largest pen collecting organization in Italy, the Accademia once organized shows and informal meetings, and published a fine quarterly magazine, Stilomania. Unfortunately, the Accademia is no more, but its website (in English) is still up and contains much useful information on Italian vintage pens, including a series of original articles by the former club secretary, Letizia Jacopini.
In addition to the above, there are a number of other national, regional, and local clubs, many of which have regularly scheduled meetings. Further information can usually be found online or through the larger national organizations.
You may find mention in older publications of the Pen Fancier's Club, based in Dunedin, Florida. The creation of pen collecting pioneers Cliff & Judy Lawrence, this enterprise began as a club but around 1980 became a retail pen-selling business. Their later catalogs were called the Pen Trading Post. Back issues of their Pen Fancier's Magazine remain a useful reference for collectors.