RUNAS - Error #3 Null

I am trying to use the following to add the logged on user to local admins

runas mydomain\adminuser password "cmd /c net localgroup administrators %username% /add"

I get this back

Logging in as mydomain\adminuser..
Setting user environment...
Error #1314 loading user profile! Error ignored
Logged in. Impersonating...
Starting process cmd /c net localgroup administrators correctusername /add...
Error #3: (null)

The user account thats logged in has no admin rights, and I want to use this as part of a scripted installation process. Ie image process finishes user logs on with credentials - this runs to add them to local admin and then a final reboot to finish it all off.
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The error indicates that the specified process couldn't be started. I suggest checking first if any process can be started, for example,

runas mydomain\adminuser password "cmd /c echo %username%"

if that works, then dig into the specific command. If that doesn't, check Windows security event log for the associated messages, perhaps they can shed some light.
By the way you don't need cmd /c to start net.exe, net.exe is a program not a DOS command.