Depending on your authentication system and organization policies, the ability to share assets with external users may already be in place.
Office 365 Users
Using Office 365 allows you to share documents and document access with external users; this setting is provided by default in your Office 365 subscription. Whatever sharing policies your organization has set on O365 will reflect accordingly on the Upteamist instance of your site.
For more information about external sharing, please review: http://bit.ly/1vmPup9