There currently isn't a way in the designer to have more than one user edit the same cookbook. Only the original owner can log in to edit a cookbook.
If you want your friend, co-worker, or co-creator to look at your design so far and do some proofreading, you can generate a print-ready proof PDF. You can print the proofs at home or share via email. These files are not to be reproduced, and there will be a watermark on the pages. Once you finish your edits be sure to regenerate print-ready files.
To generate your proofs, click "preview and order" in the blue toolbar at the top of the page. Select the binding style you prefer, and then generate your files. You can then share these PDF files with whomever you'd like to edit your book.
You can always edit your book as much as you like even after you review your files. When you're ready to order after making your edits, generate files one last time and click "approve and publish" to send to your store page.