Some users are mentioning SimpleX, which has some very good features, but for activism I'd really suggest Briar. Just scan each other's QR codes to add all the needed contacts with no real names and create a mesh network through WiFi or Bluetooth connections between devices (no internet needed). If everyone is still bent on using Signal, whoever owns an Android phone should at least download the .apk from Molly.im. This version of the app is better suited for this.