What is Guest Link feature in SharePoint Online 2013?

The Guest Link feature of SharePoint 2013 allows you to share individual documents, files and folders with out side users. You can invite people from outside your organization to collaborate and share documents. There are two ways to share documents, with or without creating user account. When you want to share individual document then you don't need to create an account but if you want to share a whole site then creation of account is necessary.

Before sharing a document you need to know who are external or outside users. External user is that user to whom you grant access to your SharePoint Online site and who is not a member of your organization. Here I am describing External user rights:

What an external user can do?

1. External users can can view and edit documents using Office Web Apps.
2. External users can see the contents of subsites also for which they are invited.
3. External users can use the rights of SharePoint Online customers who has invited them to collaborate on a site.

What an external user can't do?

1. External users are not allowed to create ID on MySite and SkyDrive Pro.
2. External users are not allowed to edit their own profiles.
3. External users are not allowed to see the company news feeds.
4. External users can't use Search service of SharePoint.
5. External users can't be an admin.

How to share a site?
To share a site you first need to send an invitation to the external user with whom you want to share your site. When that user accept your invitation, he can log in by using either Office 365 user ID or Microsoft account. You can also decide what type of permission you want to assign to the eternal user. For security purpose, create another site that you will use only for external sharing and also assign unique permission to this site. To share a site you need to have full permission of it. Follow below step to share a site:

1. First make sure external sharing feature is turned on.
2. Now go to the site that you want to share and click Share.
3. Type the email id of the external user in the Share dialog box.
4. Type an invitation message.
5. You can also assign a group to the external user, for that click Show options and select the group.
6. Click Share.

Note: The invitation that you send to an external user is valid only for 7 days, after that it will expire. You need to resend the invitation if the user does not accept in 7 days.

How to share a document?
There are two methods of sharing a document with external user one is by requiring a sign in and another by sending a guest link. To share a document you must have full permission of the site.

1. Select the document that you want to share with external user.
2. Click the ellipses...and then go to Property dialog box and click Share.
3. Type the email ids of external user or users in share box.
4. Now assign the permission to external users. For example if you want the user to only view the document then select Can View.
5. Type an invitation message.

Perform one of the following task:

• If you want to invite user by sign in method to view or edit the document then select the Require sing-in checkbox.
• If you want to invite user by sending a guest link to view or edit the document the clear the Require sign-in checkbox.

    7.   Click Share.

Best practices for sharing documents and sites

When you share your whole site with external user then that user will become the member of the site. He will be able to view or edit the documents. If you share your team site with external user then he will be able to view the contents of team site including subsites. To prevent external user from gaining access to your confidential data you can create a subsite of your team site and create unique permission for external user.

Be careful while sharing your documents using anonymous guest link because external user to whom you have send the invitation might share the link with others. So avoid sharing confidential data using links instead use sign in method.

Although this is a great feature but you should also consider security requirement and sensitivity of information before sharing.