You need to register a TechBase account before you can be added to the Translator group. Register or log in here. The workflow for on-line translators is described in Translate a Page. Off-line translation means using other tools like Lokalize and uploading ready translations here. You are not confined to translating the language you originally registered as long as your grasp of the language is sufficient, and you do not need any additional registration.
Be aware that your contribution will be governed by the Gnu Free Document License and Creative Commons License, see KDE TechBase Wiki:Copyrights for details.