In NAACP v. Alabama, the Court affirmed that the constitutional rights of speech and assembly include a right of private group association. The idea that Americans are free to join private groups was not new in 1958. However, the Court's decision to allow private groups to keep membership information confidential was an important constitutional milestone."Link (Thanks, Guilherme!)
Whether handwritten on lined paper or stored electronically in a computer system, membership data is constitutionally protected from mandatory disclosure.
The fact that technology has made it easier to collect, store and share data revealing individuals’ group memberships should be of no consequence. The principles of expressive private association, confidentiality and anonymity embodied in the NAACP case should have an abiding place in the jurisprudence of every enlightened democracy.
I write books. My latest is a YA science fiction novel called Homeland (it's the sequel to Little Brother). More books: Rapture of the Nerds (a novel, with Charlie Stross); With a Little Help (short stories); and The Great Big Beautiful Tomorrow (novella and nonfic). I speak all over the place and I tweet and tumble, too.