The plaza at 55 Water Street was too windy, the Occupy Wall Street protesters had decided, and there were too many cops. "There’s more police here than usual," said Shawn Carrié, a smiley twenty-something with a mass of small dark curls, "and it’s noisy." He gestured to his right. "And there’s a helipad over there." About two dozen organizers had assembled in the plaza but more were on the way, including some people from other cities. "Hey, we’re going to have to move," a girl wearing a backpack and purple pullover told Carrié. "Can you send a text to the whole group to let them know?"

Carrié is a member of the Tech Ops Working Group, and he’s one of the organizers with the ability to send mass texts using Celly, a tool that has aided the movement since its occupation of Zuccotti Park near Wall Street last year. Celly was developed by a pair of founders in Portland, Oregon, Russell Okamoto and Greg Passmore, who quit their day jobs in April. Essentially, individuals and organizations can use Celly to broadcast messages to everyone who has joined a group, or "cell." The terminology isn't ideal for a protest movement given that "cell" is frequently used to describe terrorist groups, but the technology is perfect for a decentralized occupation prone to spontaneous marches.

Unlike other group texting apps on the market, the number of recipients in a cell is unlimited. The main cell for the Occupy Wall Street demonstrations on May Day surpassed 6,000 members. Communication can be either one-way or two-way, with the administrator of a cell deciding who can text the whole group. The administrator can also decide whether to make the group private or publicly joinable. Members’ phone numbers are not visible to each other.

Celly offers additional functionality including polls, voice alerts, web alerts, a searchable archive, and a hashtag-esque feature called "hashlinks," and there are even more features available through its newly-released Android app. But its most basic function — sending a message to a large, mobile group — is what makes it popular.

"I really have to say that Celly is hella, hella useful, and we’d be nowhere without it," Carrié said as we walked to the second location at South Ferry, where a much larger group had gathered just outside the grayish building the Department of Homeland Security shares with the Coast Guard recruiting center. "We know that everyone... is going to pull out their phone and look at their text messages," he said. "To use the dumb marketing speak, it’s a 100 percent open rate."

The bigger cells are open for anyone to join, which means police are always listening. But for many messages, such as the early morning alert last year when police cleared the park occupation in New York, it doesn't matter if someone is listening in. For more sensitive matters, Celly is capable of creating invite-only cells.

Supporting Occupy was one of Celly’s first large-scale applications, but by now the service has gotten less radical. Celly has grown to 20,000 active cells which include businesses, neighborhoods, churches, and the city of Portland, which has cells for different bureaus. Teachers have started using the service to communicate with students, and Celly has become associated with the educational "bring your own technology" movement.

Melissa Seideman, a history teacher at Haldane High School in Cold Spring, New York, uses Celly to communicate with students and parents, which sounds like a useful strategy for teachers who enjoy being extremely accessible. "Students can text me a specific question such as ‘what is on the test tomorrow?’ or ‘what did I miss in class?’ if they were out sick," she said in a statement. "With Celly, cell phones have the potential to bridge the gap between the home, school, and social media world."

There are many popular mass texting services, but most cater to marketers who want to "blast SMS text messages," to quote one ad for Club Texting. "Text marketing helps grow sales, drive revenue and automate your business," explains CallFire, another bulk text message supplier. Meanwhile, social group texting apps like Beluga and Fast Society were faddish a year ago, but the hype died down as those startups shut down or were acquired. Apple’s iMessage service also displaced part of the demand. (The exception is GroupMe, which is still sending 550 million texts per month after being acquired by Skype.)

Celly, which is a free service with paid features for enterprise customers, has a different pitch. Celly was designed after its founders spent time talking to teachers and students, city government leaders, and activists about how they wanted to communicate and at what level of privacy. "The service was really prototyped to solve the social sharing problem in private or restricted groups and interact in a secure way without exchanging phone numbers," co-founder Okamoto said. "We really consider ourselves much closer to a private version of Twitter."

The company plans to follow up the Android app with an iPhone app in the next few weeks. For Windows and other platforms, Celly is working on a mobile website. The company is also developing ways to heighten security for users who want even more privacy, but Okamoto said he’s not ready to talk about those publicly yet. As Twitter fights to keep from handing over an Occupy Wall Street protester’s data to a New York court, privacy may prove to be one of Celly’s most compelling features.