How to create a Subsite with unique permissions in SharePoint?
Create a Subsite with unique permissions:
  1. Log into Office 365 and select your SharePoint site.
  2. Click on the Settings gear > Site contents.
  3. On the Site Contents page, click New > Subsite.
  4. On the next screen, fill out these fields, and then click Create at the bottom of the page.
    • Title and Description
    • Website Address
    • Template e.g. Team Site (classic experience)
    • Permissions e.g. Use unique permissions
    • Navigation
  5. Add members to the site groups:
    • Users added to the Visitors group, will only able to log in and read what is on the subsite
    • Users added to the Members groups, will be able to contribute information (edit) and add documents to the site
    • Users added to the Owners group, will be able to add and delete pages and components within the site
  6. You can always add more users and/or edit their access to the site later.