http://efa.iykpqm7jiradoeezzkhj7c4b33g4hbgfwelht2evxxeicbpjy44c7ead.onion/toolkits/building-coalitions
Try sending emails, and count on phone calls for crucial follow-ups. Don't overdo it with too much bugging. In emails, provide clear links to the current draft of the letter and the registration sign-up; in email threads, you may need to remove links to old drafts. Always approach large organizations early.