Afaik you will need to add the user to the root group as well to have access to the DCUI
New-VMHostAccount -ID MyUser1 -Password MyPassword1 -UserAccount -Server MyESX1 -AssignGroups root
From ESXi 5.1 onwards you can also do this with a non-root account by adding the account to the DCUI.Access advanced setting.
Something like this
New-VMHostAccount -ID MyUser1 -Password MyPassword1 -UserAccount -Server MyESX1
$dcui = Get-VMHostAdvancedConfiguration | where {$_.Name -eq "DCUI.Access"}
Set-VMHostAdvancedConfiguration -AdvancedSetting $dcui -Value "$($dcui.Value),MyUser1"