You can't restrict tax rules based on user groups.
I am trying to make a certain user group (which I have named "Reseller") exempt from the tax rules.
I tried to follow instructions that I found in a different post regarding tax exemption, and the advice was to create tax rules for the other types of customers (individual, company without VAT) and then leave no tax rule for the one you want exempt. That didn't work for me because I couldn't see how to designate a customer to one of those categories.
The custom types that you can configure in your tax rules are based on the "company" and the "vat number" fields that you can enable via the menu Display->Custom fields.
If a name is entered in the "company" field, the user will be regarded as a company, with or without VAT number based on the vat number field. and if the company field is empty, the user will be regarded as individual.
So it's really simple to setup.