unfortunately that's not possible... the people who want to contribute will need to have a password
when the forum was open to anybody to post I spent a quite a bit of time cleaning spam..
having said this... we are about to launch a new arduino website (on the arduino.cc domain)
I'm happy to have a wiki where anybody who asks can have a password.
since the main site is also a wiki we could promote to the main site any page written by users that makes a significant contribution
massimo