The San Diego Tropical Fish Society is pretty active. It meets in the evening on the second Sunday of the month in Balboa Park. That said, it doesn't provide much support to the North County. The Sunday evening meetings make it difficult to attend when there is any travel involved and they make for very late work/school nights when one comes home with fish or plants to be attended to. The folks are nice, but it's also an old club, and members are pretty resistant to changes that might attract new members, and the speakers are really variable in quality. As a result, I've let my membership lapse and plan to join COAST instead (further, but meetings are at a much more reasonable time). Despite my criticisms, anyone in central San Diego would be well-served to check out SDTFS as there are lots of great deals on plants and fish at the auctions, and the membership is very inexpensive. It's also a great place to meet other hobbyists and find new resources. I just can't justify the travel and late nights even with the benefits.
I've kicked around the idea myself of planning meetings in the North County and maybe arranging trades or group buys (or carpools to OC or COAST meetings). I would be up for attending meetings (maybe at a restaurant or something to start) so feel free to message me. I'm happy to take on some of the organizational duties.