Final answer:
The approach by advocacy groups that uses voluntary media and advertising industry participation to spread their messages is called outside lobbying, a form of grassroots lobbying that taps into public support.
Step-by-step explanation:
In the context of advocacy group tactics, the approach that relies on voluntary participation by the news media and the advertising industry to disseminate an advocacy group's message is known as outside lobbying, also referred to as indirect lobbying or grassroots lobbying. This tactic harnesses the power of public opinion and involves activities such as issuing press releases, securing media coverage, and prompting the public to exert pressure on lawmakers.
Advocacy groups like the Sierra Club may use outside lobbying to publicize environmental concerns and encourage citizens to reach out to their representatives. By capitalizing on the reach and influence of the media and advertising industry, interest groups can effectively spread their message and rally public support without direct expenditure on advertising.