Powers Vested on the SharePoint Online Site Collection Administrator
As a Site Collection Administrator, you have the highest level or permission in your site collection that allows you to perform tasks, which may have routed to your IT team in the past. You also have the power to enable or disable features that are used in subsites within your site collection.
Moreover, you have access to all the subsites regardless of whether you’ve been added as user to the site.
How to share your SharePoint Online site externally
When the SharePoint Online Administrator enables external sharing from the SharePoint Online Administration Center, this does not mean that all site collections are automatically shared externally. You, as a Site Collection Administrator, have two more steps to take before you can start inviting external users.
Activate the site feature.
From the top-level site in your site collection, go to Site Actions→Site Settings.
Below Site Collection Administration, click Site Collection Features.
Click the Activate button to the right of External user invitations.
Share the site (or any subsite).
Go to Site Actions→Share Site.
Enter the e-mail address of the external user you want to invite to the site and then click Share.
If the external users you invited to your site already have an Office 365 account, they can log in by using that account. If not, they will be asked to log in with a Hotmail, MSN, or Live account.
How to create a new SharePoint Online team subsite and/or new document libraries
A subsite is merely a SharePoint site under a site collection. It uses the same navigation as the top-level site and has the capability of using all the site collection features that have been activated at the site collection level. Sometimes, it is referred to as the child site, whereas the top-level site is called the parent site.
To create a subsite, follow these steps:
Go to Site Actions→New Site.
The Create Window appears.
From the Silverlight carousel displayed in the Featured Items tab, choose a template by clicking on the images in the carousel.
Enter the title of your site in the Title box and enter the URL for your site in the box below if you want a URL different from the title.
After the request is processed, you are taken to your new site.