When President Donald Trump arrived in Nashville on Mar. 15 to stump for his new health care and school reform polices at the city’s Municipal Auditorium, he was greeted by something unexpected. Several hundred protestors waving signs were already marching through town against him.

That uprising wasn’t really supposed to happen. Anti-Trump forces had learned of the trip just a day or two earlier, and on short notice, grassroots organizing is difficult: Rallies generally take time plan and have to come together through lots of decentralized chatter. (There’s the flurry of Facebook and web-page posts, phone trees, even fliers). The Nashville effort, however, was plotted differently. An organizer tagged the upcoming event to a map within the a new political action platform called People Power. As others learned about it, they RSVPed their interest and were kept in the loop via whatever kind of communication they preferred, including up-to-date blasts from text messages.