A user with Administrator Rights can add modules and change the licensed configuration in Account Settings in the top right dropdown menu. In order to add a module, go to the first tab named Current configuration, change the number of extra modules needed and click the button on the lower right corner to update the configuration.
The system will automatically track the days that each extra module has been active and you will be billed accordingly for the usage.
RunMags comes with unlimited free users and the licensing is set up so you only have to pay for functionality you use.