Sharepoint 2013 People Picker

The People Picker in Sharepoint 2013 is a bit slicker. 

Instead of having to click on search, start entering a name, finding the person and then adding you now just have to start typing the name in the box. As you type a dropdown is created of people who have those letters in their name, which gets filtered as you type. Simply click on the required person from the dropdown to add them.

Click on Show Options and you can then select a group or permission level.

The user does not exist or is not unique

One of the web front end servers in our farm was throwing an error when users were being added to a Sharepoint group. The error displayed was the rather generic User does not exist message so I went sniffing in the logs.

Using the Correlation ID I found what was causing the exception,

An exception occurred in AD claim provider when calling 
SPClaimProvider.FillResolveClaim(): Requested registry access 
is not allowed.

A search on this error led me to Bernardo's excellent blog on the problem. I followed his steps and found that my app pool account was indeed being refused access to the registry key

HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Shared Tools\
Web Server Extensions\14.0\Secure

I added the App Pool account to the WSS_Restricted_WPG_V4 instead of the local admin group on that server, it worked fine and resolved the issue.