Microsoft change the interface so often that it is difficult to keep documentation up to date sometimes.
A common query that we get is how to view the external users in the admin portal/interface rather than having to resort to powershell.
The answer as of Jan 2017 is quite straight forward.
1. Go to the Sharepoint site you want to administer.
2. Click on the cog in the top right and choose Site Settings:
3. Choose Access Requests and invitations:
You'll be able to view the external users that have pending requests with the access they've requested.
Also you can see under History, the users that have accepted or declined invitations.