I have an urban warfare situation, a militia group invading a small town and being fought by the residents. The residents have very little time to prepare for the invasion. Ten minutes, maybe. Assuming I can find a way to notify them all of the invasion, what's the most effective way for them to communicate with each other during the battle? Ideally I'd want them to have instantaneous mass communication, like radios that could broadcast instructions, and if it was useful I could make sure the town has a radio station that could be commandeered for the purpose. But... I don't have a portable radio. I don't think most people have portable radios these days, at least not ones that aren't hidden under an hour's worth of junk in the attic. Is there a way to make cell phones, or some similarly common device, work for this? I want something more mass-directed than just calling everyone or sending them texts. I was thinking about a Twitter hash tag, but that seems a bit non-direct. Can anyone think of a way for a group of modern citizens to stay in touch with each other as a group during a crisis situation?