"If I contribute as part of my job, is it legal for me to sign away my copyright (ie in might be my employers copyright in the first place and they might have opinions about giving away their copyright)."
I scanned over the text of one of the documents and what struck me was that the contributor represents that he has authority to transfer the copyright. That means that if you're wrong, they're protecting their ability to sue you. Such things are boilerplate in rights transfers, but what, exactly, are you getting out of the exchange for potentially opening yourself up to more or easier liability?
With that clause in there, unless there's no doubt about your rights in the work (i.e. you're unemployed and a hermit), a prudent person should think twice about making such an open ended representation without any compensation.