Why do I have to authenticate to the domain to add a local user to a local group?

I login to a Windows 2008 Server as Administrator and when I try to add  a user to a group, it makes me authenticate to the domain before I can do so.  

As soon as I click the Add button, I get a dialog containing the following message:

Windows Security
Enter Network Password
Enter the name and password of an account with permissions for <name of domain is here>

So I enter my network credentials and it allows me to add the user.

If I login to the server with my domain account, it doesn't prompt me because I am already authenticated to the domain.  If I were modifying a domain group, that would make sense to me. But this is a local group defined on the server, not on the domain.

Is there a setting that controls this?

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Rich RumbleConnect With a Mentor Security SamuraiCommented:
It's because the local account doesn't have the ability to connect to the domain to query the domain to look up the name your adding. You could use any domain account for this, any account that isn't locked out that is.That is why your prompted, the local account your using can't authenticate to the domain. It's the same as if your logged in as a local account and attempt to connect to a share on another computer, if the same local account and password isn't present on that domain pc, you'll be prompted to enter a domain account.
Although you haven't stated how you manage security issues on this server, Windows Security Authentication in Windows Server 2008 dates back to 2003. The following should help you.

MajorBigDealAuthor Commented:
Thanks Rich,  after reading your comment, I tried cancelling the dialog but continuing to add the user. I am able to add a local account without authenticating to the domain, I just have to cancel the dialog first.  Of course, when adding a domain account it doesn't work unless I authenticate first.  

I was just confused because I was assuming that the dialog would not come up unless I actually needed to authenticate. Bad assumption.
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