Is it worth attending the meetings for these clubs typically
Community is one of the reasons for joining an investment club. I would say that not attending the meetings would be counter to the whole point of being a part of one.
Are they just fronts for sales operations? (Many are run by real estate agents.)
Not necessarily. That said, I would be wary of ones run by real estate agents, insurance agents, etc...
Is there a sanctioning body that accredits or oversees these groups?
I do not think so. Though The Motley Fool mentions the National Association of Investors Corp..
The Motley Fool has a decent article on investment clubs. I think it will help answer many of your questions. They go into forming one, what to look for in an investment club and potential issues.
They stress that investment clubs are about community, having fun, making investing accessible to the novice investor, pooling money for investment and making investing accessible to those without a large amount of money to invest. They also point out that it can help reduce the work load of researching investment ideas. Finally, they also state that organization is important to having a successful investment club.