Each have as a central thread the appearance of being connected to occultism; but some are definitely far more occult than others. Some groups are actively full of magicians, some are fronts for Mythos organizations, some are outright frauds, some are full of useless poseurs, and a few are conventional "secret societies" or fraternities with only some light occult element (but a lot of connections of importance to the setting of Arkham in 1910).
Today's group is mostly one of the latter. The Progress Club is a secret society only in the sense that no one but members are allowed to attend the club, and in the sense that the members do not talk about what they do in their meetings. But everyone in town knows where they meet, everyone knows what they sponsor (with the charities they started and buildings they've paid to construct having their logo, in the form of an arrow pointing upward), and members are often very public about their membership (wearing an arrow pin on their lapel).
The Progress club is a social club for the richest members of Arkham's rising "new-rich" Industrialist class; the wealth, progress-minded factory owners and venture capitalists that in many ways reflect the zeitgeist of the American facet of the Edwardian age. They're Arkham's answer to the Rotarians or the Lions' Club; a private society for the very wealthy and influential to get together, network, engage in charitable acts of philanthropy for the town's betterment and to improve their own public image while at the same time trying to control the local city politics.
In Arkham, however, just about every public group has some kind of hidden occult underside; in the case of the Progress Club, it is in reaction to the group run by the Industralists' political rivals, the "old families" of Arkham, who have been decidedly unwelcoming to these brash new-money types, and are whispered to be very much entrenched in weird occult practices (not least through their own secret society, the Lamplighters).
The Progress Club will be the target of countless rumours around town of Illuminati-like secret (and usually sinister) occult activities. Just how much of these rumours are actually true is unclear (until you read the sourcebook, that is!), but it is known that the Progress Club has been known to act, collectively or through its individual membership, as patrons to agents (like the PCs) in investigating and opposing the occult schemes and activities of the Lamplighters and the various scions of Arkham's founding families; a task they see as just part of their multi-front secret war to drag Arkham out of the obscurantism of pre-industrial backwardness and into the modern age of progress, science, and especially big-money capitalism.
That's it for today's update, stay tuned for more!
Currently Smoking: Masonic Meerschaum + Image Perique