as others have said Promote the hobby, not the club. Here is an idea:

Offer a subscription to the Club's magazine, so people who are interested/curious about the hobby can find out more about our world.

A lot of people like to look and "Test Drive" a hobby before joining something. (I collected stamps for years before joining the APS)

Then, offer an incentive. Offer a portion of the subscription cost toward membership in the Club. People LOVE getting rebates.

There are a lot of hobby magazines that don't require membership to get their magazine (Coin World, Coin Prices, Coinage, Lynn's Stamp News...I could go on. It is just my 2 cents on the matter.