Invitations can be used in a number of ways, including rescheduling a meeting, session, or date.
To reschedule a date, simply click on the date you're looking to reschedule and click the "Edit" button. You can find the date either in the Lead/Shoot itself in the Dates section, or by clicking Overview > Calendar and finding the date there.
Click the "Reschedule using Invitation" button on the right side and choose the Invitation you're rescheduling the date with. If you don't have one, you can create a new one by clicking the "Create New" button.
Once your Invitation is set, the date will change to "Waiting on Client". You can click on "Update + Email" to save your changes, and email the request to reschedule the date to your client.
To find your outstanding Invitation requests, go to Overview > Calendars and look next to the "Add New" button. Click the "Invitations Waiting" button to see all outstanding Invitation requests.