If everyone is using iMessage, you can leave the group [0]. Unfortunately, you can't leave if someone is using SMS.
[0] http://osxdaily.com/2014/09/23/leave-group-message-chat-ios/
[0] http://osxdaily.com/2014/09/23/leave-group-message-chat-ios/
A friend of mine was in a business meeting on the same day UND announced its new logo and his phone went crazy. The person he was meeting with asked if he should answer it and he said "no, its a lot of bitter, bitter people and UND has no taste.". He did get the contract though.