Contributors to GNUnet with Git access must sign the copyright assignment to ensure that the GNUnet e.V. --- Taler Systems SA agreement on licensing and collaborative development of the GNUnet and GNU Taler projects is satisfied.
The agreements ensure that the code will continue to be made available under free software licenses, but gives developers the freedom to move code between GNUnet and GNU Taler without worrying about licenses and to give the company the ability to dual-license (for example, so that we can distribute via App-stores that are hostile to free software).
Minor contributions (basically, anyone without Git access) do not require copyright assignment. Pseudonymous contributions are accepted, in this case simply sign the agreement with your pseudonym. Scanned copies are sufficient, but snail mail is preferred.