In order to invite participants to schedule a time on your calendar, it has to be active. After you've finished scheduling participants on a specific calendar we recommend that you deactivate it (though this is not required).
How to Activate or Deactivate Your Calendar:
Click the project name to view the project overview
Click the Calendars tab
Click the options button next to Edit on the Calendar you wish to Activate/Deactivate
Select either Enable schedule or Disable scheduling