Does anyone know a way to make this module work exclusively for a customer group?

:)

You cannot configure the FreeShippingTeaser module to work only for a specific customer group. The module works with the default OpenCart shipping logic and functionality where there is no option to configure a certain shipping method to be applicable only for certain customer group.

However, the required behaviour of the module can be achieved by customizing the module functionality in order to show the free shipping notification and apply free shipping only for a certain customer group. Please mind that the default OpenCart free shipping method should also be modified in order to be applicable only for a specific customer group.