You always have short term hobbists, Life is cycles. Brewing in your twenties is fine, until the kids come along. Great in your forties, unless your knees give out, etc. You need to constantly bring in new bood. Question is, what are you offering? Used to be tht was the way to learn how to brew. I learned from the internet and experimentation, there are tons of info for free.
The brewing event is a good way to attract interest. Group brews are nice, but rather hard to do. having club hardware is nice, but expensive. I have heard of group buys. I was interested, but the person talking about them was too fast to wave me off with "no, no, that is just for members" rather than a "would you be interested in joining?"
And recruiting is everyones job. It takes a positive attitude at all times. If you are talking about brewing in jpubllic, be ready to include strangers in the conversation. If you only want your personal friends in the club, it will be a small club.
And sometimes even stupid stuff works. Get club shirts that say "Ask me about brewing, name of brew club" and wear them to large events when you are going.