Don't use "group messages" to promote your business or mass message customers!
Here is my response on a heated thread discussing this very topic today:
"In my experience, unsolicited group messages are ALWAYS a bad idea. It's one thing to do a group message to plan an event after you've talked about it over FB or coffee. It's something else to send a message to 100+ people at a time. Even if people are interested in the subject, they almost ALWAYS get pissed off by their notifications "blowing up", then the questions from people who don't understand why they are getting messages from people they don't know. Then the "how can I get removed" and the responses, then the notifications that so and so left the conversation. 1 group message to 100 people can easily turn into 500 notifications. Many people get notices by email or text. This pisses people off.
Having said that - the majority of the time that someone uses a group message, they have NEVER been part of one before, so they have no idea what to expect! I've never been included in one that was intended to be rude or insensitive, it just sometimes seems that way."