However, you should be cautious when using this feature. Sharing any site within a Site Collection enables the external user to see everything in that Site Collection, not just the sub-site you think you're sharing.
Microsoft recommends creating a new Site Collection and sharing that with External Users if you don't want them seeing internal information.
To create a new Site Collection in SharePoint Online:
- Navigate to https://portal.microsoftonline.com/ and sign in with your administrator account.
- Click Manage under SharePoint Online.
- Then select Manage site collections on the next page. From here you will be taken to the administration center:
- Select the New dropdown, the Private Site Collection.
- Once this site collection is created, it can be shared using the Site Actions menu.