Unable to add user AD account into confluence-administrators (internal) group

We are using Confluence 5.5.3, with User directory link to AD via Crowd Server as the second User Directory, Confluence Internal Directory is the first one.

confluence-administrators is NOT a group in our AD. So it should be internal Confluence's group. I'm the only user in the confluence-administrators group by now, maybe because I was the one who installed confluence.
Now when I want to add another user into this group, as a back up admin, confluence doesn't report any error, but no new account was added.

I went to Cog -> User Management, and on the left side bar clicked on Groups.
In the Find Group box, I typed in confluence-administrators, found the group and clicked on it.
From this group's page, I saw only my account there.
I clicked on Add Member, and either typed in the new account name from AD directory, or clicked the search icon to find the new account I wanted to add. then clicked Add.
The screen went back to the Group members: confluence-administrators page, but no new name was added.

I found the error in atlassian-confluence.log
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Caused by: com.atlassian.crowd.exception.ApplicationPermissionException: Group 'confluence-administrators' does not exist in the directory of the user and cannot be added.
at com.atlassian.crowd.manager.application.ApplicationServiceGeneric.ensureGroupExistsInDirectory(ApplicationServiceGeneric.java:1214)
at com.atlassian.crowd.manager.application.ApplicationServiceGeneric.addUserToGroup(ApplicationServiceGeneric.java:1115)
at com.atlassian.crowd.embedded.core.CrowdServiceImpl.addUserToGroup(CrowdServiceImpl.java:710)
... 327 more
2015-04-07 10:42:22,137 INFO [read-only-transaction:thread-1] [atlassian.confluence.user.DefaultUserAccessor] getUserNamesWithConfluenceAccess Could not find group configured with USE permission: Engg IT Admin
2015-04-07 10:42:22,137 INFO [read-only-transaction:thread-1] [atlassian.confluence.user.DefaultUserAccessor] getUserNamesWithConfluenceAccess Could not find group configured with USE permission: Dell Networking
2015-04-07 17:47:33,689 WARN [AtlassianEvent::CustomizableThreadFactory-1] [confluence.mail.notification.DefaultNotificationsSender] call Send Notification: Not sending email [ Development Operations > NEXTGENDBs For ORACLE ] to [ Hung Nguyen ]: No email set
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Why the add member action tried to find confluence-administrators group in crowd and not in internal directory first?
Can we add a user's AD account into internal directory?