Ask the group to get the insurance company to quote what additional cost would be involved to extend cover to you. Policies can be a few hundred quid cheaper if you accept a min hours requirement, which this group may have done. I think we paid around a £200 premium to remove all minimum hours requirements from our policy, which now simply states 5 hours on type.
Paying the additional premium to lower the hours requirement to 70 is likely to be a hell of a lot cheaper than hour building to 100!