Can not add Domain User to Local Admin Group Win8.1

Question

Hello, 
I am trying to add a domain user to the local admin account on a Win8.1 Enterprise computer. When I click the check name button it asks me to enter network credentials even though I am signed in to the computer with a domain admin account. When I try to
type in any of my domain admin accounts it says "The Username or Password is incorrect". Even though I used that same account to login with. I can successfully ping all 3 of my DCs from the computer and have tried putting my second DC as the primary
DNS and my third DC as the primary DC and same problem. I have checked for Active Directory errors on the DC and everything says it is running fine on the DC in server manager. I have this problem on multiple computers. Some of the computers it will work on
but 90% of them it won't allow me to add the local user to the local admin group. 
DCs are running Win Server 2008 R2 Enterprise. 
Any help would be greatly appreciated. 
Thank You

Answer 1

I would suggest you to use Restricted Group(via GPO) to add domain users/group to a local admins group 
1)Create a new group in Active Driectory
Create a new group in Active Driectory that you wish to add to every workstations local administrator group. DO NOT add any users to this group at this time.
2.
Create a new GPO
Create a new group policy object and link it to the desired OU. Make sure that the GPO you are using covers the OU that the WORKSTATIONS you are wanting to give users local administrative rights over.
3.
Edit the newly created GPO
Navigate within the newly created GPO to Computer Configuration -> Policies -> Windows Settings -> Security Settings --> Restricted Groups
4.
Add your new Active Directory group to the Restricted Group
Right-click the Restricted Groups folder and select "Add Group" to add your new Active Directory group to the Restricted Group. In the Group field, type the name of the newly created Active Directory group and click "OK"
5.
Add the Restricted Group to the local administrator group
In the Restricted Group Properties windows click "Add" under the section titled "This group is a member of:" Type "Administrators" (without the quotes and yes it is plural), in the Group Membership window and click "OK"
6.
Wait for GPO updates to apply to the workstations
Once your users receive their updated group policy settings every workstation within the OU you specified will have your new Active Directory group as a member of the local administrators group. If you need to force the GPO update on a specific workstation,
run "gpupdate /force" in a command window on that workstation.
7.
Add a user or group of users to the Active Directory Restricted Group
When you are ready, or in a position where you need to provide local workstation admin rights you can simply add the users or group of users to the Active Directory group that you created for use with Restricted Groups within your Active Directory Management
Console.