If the goal is numbers, then broadening the topic to include watergardening, turtles, goldfish, etc. will help, but it may just chase away the kichi. When the focus is kept on koi, there is the challenge of presenting programs that are not too technical for the majority, while not boring the more experienced. And, no doubt, in the U.S. anyway, the typical two-career couple putting in 50 and 60 hour work weeks have little time for much more than some quiet time by the pond ... which is why they built it in the first place.
What every club needs is a handful of folks who enjoy the tasks of maintaining an organization ... newsletters, collecting dues, drafting folks to host meetings,etc., etc., ... and is a lot easier if some of them do not have regular jobs.And, it really does not matter too much if they are kichi. The sane ones sometimes keep their perspective.
For myself, I just like being around my local "koi friends", hearing what they've done, what they plan to do ... and seeing how their koi have grown. Even when I'm rushed trying to squeeze everything that needs to be done into a weekend, the meeting gets some priority.
What will draw folks?... programs with lots of photos of koi and ponds.