If owners are working on own property or common property they will not be considered employees unless you pay them.
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I normally represent injured workers....not employers but I wold say that non-owner volunteers (ie. Boy Scout clean up days) would be covered. Owner volunteers would probably not be. Check with your insurance carrier for a more definitive answer.
More info needed. If the volunteers are working on a regular basis, they are probably entitled to workers comp benefits if they become injured while doing their volunteer work. If you have insurance, you should read the policy.