Head over to your InsideSales tab in Salesforce, and click the License Assignments link in the User Management heading. Find the PowerStandings Pro checkboxes in the employee list, and start selecting the users you want to have a PowerStandings license.
Remember you can filter the employee list by name or role to hunt down specific users.
If you don't have enough available licenses (or any at all), head to the Manage License Quantities link on the InsideSales tab, and add more.
NOTE: Additional licenses will automatically be added to your ongoing billing cycle.