Assuming you are referring to a TRUE group plan, the biggest obstacle is finding a carrier willing to offer on such a group. Most association plans now are written with individual policies, not group.
Most insurance companies do not like to work with independent contractor. At least that is the case is California. I tend to think that association group plans are way to overpriced. The reason is that most people that end up staying on those plans have medical issues and cannot qualify on their own. I know in California association group plans are just money drain for the insurance companies. If insurance companies could cancel them all they would. The current contracts do not let them. It is much easier for a Realtor just to get a individual plan (if she or he can qualify).