The server is set to run scared if it goes beyond a load of 5.
I do not see a way that can be changed by user group.
vBulletin can read the overall load of the server on certain *NIX setups (including Linux).
This allows vBulletin to determine the load on the server and processor, and to turn away further users if the load becomes too high.
If you do not want to use this option, set it to 0.
A typical level would be 5.00 for a reasonable warning level.
I do not see a way that can be changed by user group.