Yes you will need to register, but it is free.
Sounds like you want to start a tree based on an existing one. You should use the [Download / export this family...] button at the lower left to download a GEDCOM or FamilyScript backup.
Switch to your account and in the My Account screen you can create a new tree. You will then have the option to upload the file you saved. Once uploaded be sure to set the "me" to your entry to complete the upload.