The disadvantages of being part of a newsgroup is that you will constantly be harassed by advertisements. There is also the risk of receiving viruses on to your devices. They could also pose a security risk for instance the person on the other side of the message may not be who they say they are harassing you or asking for your personal details. The details you received might not be accurate such as the product someone is selling might not turn out as it was described.
The way business and individuals can protect themselves are by installing anti virus software. Blocking out unwanted visitors and double-checking you are actually talking to right person if you are not sure for example by ringing the company. Also by checking on official websites and shop that the product you intend to buy is right for you and does what has been said.