Have the document open, then Tools>Protect Document. You then get a section which allows you to protect what you want. In my case, in the box in step 2, I chose to allow comments to be edited.
In Exceptions, I ticked Everyone, then in step 3, Yes, start enforcing protection. You then get the chance to enter a passsword and that's it. Anyone can add their details but they can't change the logo or the footer.
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