When you store your files in the cloud, you can easily share files from a SharePoint team site, or from your One Drive for Business.
In the OneDrive for Business or the site library right-click the file you want to share and select Share.
Type in the name(s) of people you want to share the file with, and then select permissions.
For other ways to share files and folders, see Share documents or folders in Office 365.
On a team site, on the upper right corner, choose the Share icon, and then type in names or emails of people you want to share with.
Yes, sharing sites and files externally is very similar to sharing internally. You just type in the external email address, instead of an internal one. For details, see Share SharePoint files or folders in Office 365.
Note: If your admin has turned off external sharing you will not be able share with people outside your organization. See turn external sharing on or off for details.