With how helpful the staff has been to me as a new customer I just felt like giving something back to help out others here.
Step 1:
If you log into your Killing Floor Web Admin Interface you'll notice that the Admins & Groups option is disabled (If not disabled then your all set to add users, and clan members as admins already). The image below shows the link disabled.

So lets get started on enabling your Admins & Groups controls. Log into your NuclearFallout Control Panel. Go to your Killing Floor server and click File Manager. Now select your Killing Floor config file (default: KillingFloor.ini) and then select Edit for the option in the 2nd drop down control. Now click the Submit button.

Step 2:
Your config file should now be opened in an online editor within your control panel. Look for a line called AccessControlClass=Engine.AccessControlClass under the [Engine.GameInfo] section.

Now replace / rename the line called AccessControlClass=Engine.AccessControlClass to AccessControlClass=xAdmin.AccessControlIni and then click the Save button.

Step 3:
With your config file done, you will need to go to your Server Control within your NuclearFallout Control Panel for your Killing Floor server where you will do 2 steps.
Step A: Add ?AdminName=XXXXX?AdminPassword=XXXXX to the end of your starting line. Were the XXXXX are; put your Admin Name and Password or you won't be able to log into the Killing Floor Web Admin Interface, even if you already have your Admin Name and Password inside the config file already.
Step B: In order for this to take effect you must restart your server. From the drop down box select Restart the server and then push the Do it button.

Step 4:
Time to see if it worked. Log into your Killing Floor Web Admin Interface and you should now be able to access the Admins & Groups control. You can now add new admins from this page. You can limit the admin powers they get and assign them login information to sign into a sub account of the Killing Floor Web Admin Interface panel for controling and monitoring the Killing Floor server.

Good luck and hope this helps

Scott,
PS: I would of added the images as attachments but I was only allowed 3. So if you do add this tutorial to your killing floor help, could you re-upload the images to the NFO site.