We started 10 years ago with offering a plan the covered about 8-10 employees initially. As mentioned, they will rate individual members (and their family members) at this level (or used to, we've grown and I am not involved in the process anymore). I got a nice little report during the quote process that showed by age and sex what was driving the costs. Then in year 2 I got same report that also included claims. You could sort out who was driving the costs shared by the entire group. You have to have the mental fortitude to ignore this if you want to keep family and numbers separate.
Its a fact of underwriting, but young single males will drive your costs lower as they rarely go to doctor, do not get pregnant, and generally have less health concerns. We are in construction, so our demographic trended this way naturally. That being said, you can influence participation rates on your plan. This is done by offering the right plan options that may get those folks that drive your costs down to sign up and also by adjusting your employer contribution. When we began, we paid like 75% if employee only, then 50% on spouse and family. We also offered a lower premium, high deductible option. Made it a no-brainer for those young males to sign up, which drove the costs down for the group.