To add an author to Squarespace blog, you can follow these.
#1. First, you choose Settings
#2. At Permissions & Ownership > Click on Permissions & Ownership
#3. Click ADD BASIC AUTHOR
#4. Enter some info then click Save
#5. Done. You will see the result.
#6. To assign an author to a specific post, you can hover on a post > Click 3 dots > Click
#7. And do it here.