How volunteers are handled differs with different types of insurance. The general liability policy does include protection for the actions of volunteers; Worker's Compensation on the other hand does not include volunteers.
In the standard general liability policy, which most businesses use commonly, names volunteers under the definition of who is an insured? Who is an insured is important, because it names who the insurance company will defend against a lawsuit or other legal action. Volunteers are specifically listed in the most commonly used General Liability language.
Worker's Compensation is more nuanced, but seems to have been well established by the courts. Coverage begins with the definition in Massachusetts laws: chapter 152 is the Massachusetts law that addresses Worker's Compensation definitions and other features. Section 4 of that law defines employees as those persons under contract or higher, with exceptions for certain categories such as real estate agents, consumer products sales people, cab drivers, and others specifically excluded. Clarifying further the "contract for hire" section, in Lowery v. Klemm, 446 Mass 572, 580 (2006),
the Massachusetts Supreme Court wrote that "Worker's Compensation statute does not apply to volunteers... "
This volunteers appear not to be considered employees.
Coverage for those ordinarily excluded as mentioned above, such as real estate agents, cabdrivers, and consumer products sales people, is available with a specific endorsement, but this does not apply to volunteers.
These categories notwithstanding it's easy to get conflicting interpretations from the state. The Department of Health and Human Services posts an on-line brochure stating that volunteers do have Workers Compensation coverage, but the Department of Industrial Accidents and the Worker's Compensation bureau disagree. Given that the departments that oversee Worker's Comp directly are on the ‘no coverage’ side of this equation, we expect claims made by volunteers would not be honored.