Volunteers doing work on behalf of your business are typically covered under your general liability, but you need to check with your policy or your agent to make sure.
Subcontractors should provide you a certificate of insurance showing general liability coverage and name you as an additional insured on your policy for maximum protection. In some states, you should also have your subcontractors show proof of workers compensation. Otherwise, they could be lumped in with your other employees at audit.
I hope I answered your question.
Brent Kelly, CIC