Cannot add users in crm using "Add Multiple Users"

Please see below for the solution :
If you do have more than one, please add a String: "PreferredDC" to the MSCRM hive in the registry on the CRM Server (HKLM - software - Microsoft - MSCRM) Add in the FQDN of the domain controller within your environment which holds the user domain accounts.

Change the Search order in the registry of the CRM server and point to Your Primary Domain Controller. To do this make the following registry change: HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters]"SearchList"= DCName. Change the "DCName" to your Primary Domain Controllers name You should use the FQDN of the Domain Controller where the DNS Server is held and where the CRM Domain accounts are. You MUST reboot the CRM Server in order for these change to take effect.