You don't need to register not for profit organisations, though there are benefits in some cases for charities with official status. Unincorporated associations don't legally exist in the same way that limited companies do (i.e. there is no limitation of liability) but they are not illegal.
"If youíre not using valid HTML, then you havenít created a Web page. You may have created something else, but it isnít a Web page." - Joe Clark