There are 2 ways to share your unpublished Squarespace site:
Option 1: Set your site to password-protected and send the password to the person you want to share.
#1. First, choose Settings
#2. Next, choose Site Availability
#3. Choose Password Protected then enter your password
#4. Click Save on the right of the screen
Then share your site with the password
Option 2: Change the site to private then add Contributor or Viewer permissions
You can follow this guide to add Contributor or Viewer Permissions