The design and use of Information and Communication Technology (ICT) has now evolved beyond its workplace origins to the wider public, expanding to people who live at the margins of contemporary society. Through field work and participatory co-design with homeless shelter residents and care providers we have explored design at the common boundary of these two "publics." We describe the design of the Community Resource Messenger (CRM), an ICT that supports both those in need and those attempting to provide care in a challenging environment. The CRM consists of three components: 1) a message center that pools messages to and from mobile users into a shared, persistent forum; 2) a text and voice messaging gateway linking the mobile phones of the homeless with the web-enabled computer facilities of the care providers; 3) a shared message display accessible from mobile texting, voice, e-mail, and the web, helping the two groups communicate and coordinate for mutual good. By democratizing design and use of technology at the margins of society, we aim to engage an entire "urban network," enabling shared awareness and collective action in each public. © 2010 ACM.