Share Site with an External User

An external user is a person who has been granted access to your SharePoint Online site, but who is not a licensed user within your organization. External users are users who are not employees, contractors, or onsite agents for either you or your affiliates.

You can grant access to external users by using the Share command to send them an email invitation to a document or site. For more information about how external sharing works, see Share sites or documents with people outside your organization.

For more information please refer below articles:

http://office.microsoft.com/en-in/office365-sharepoint-online-enterprise-help/what-is-an-external-user-HA104036809.aspx

http://office.microsoft.com/en-in/office365-sharepoint-online-small-business-help/share-a-site-with-external-users-HA101850586.aspx