How to configure external sharing in SharePoint Online

As an Enterprise plan admin you can configure external sharing at two levels:

• SharePoint Online environment (tenant);
• individual site collections.

To enable external sharing on the site collection level you should check whether the feature is enabled on BOTH tenant and site levels:

Configure external sharing for your SharePoint online environment:

1. Log into the Office 365 portal, click Admin->SharePoint to access SharePoint admin center.
2. Click Settings, for the external sharing section, please check whether external sharing is enabled.

 Check for individual site collection:

1. Go to the SharePoint admin center, click site collections.
2. In the Site Collections list, select the specific site collection(s) you want to share.
3. Click Sharing to enable the external sharing.

And now you have possibility to share the site. Please, choose "SHARE" and fill out the table:

NOTE: If you decide to disable external sharing for individual site collection later, existing external users will not be able to access content inside the selected site collections. If you choose to enable it again, you may have to re-invite external users. If you want to temporarily limit external user access, consider turning off external sharing from the Tenant Settings page instead.

For more information, please refer to this article.