You can add custom notes to all of your meeting invitations for both Ad hoc meetings and different Meeting Templates.
Ad hoc meetings
In your Ad hoc scheduling preferences, scroll to the section called “Invite Notes”:
Add the text you would like to include on all of your Ad hoc invitations in the text box, and they’ll be included on all of your Ad hoc invitations.
From your Meeting Templates preferences, click the “Edit” button for a specific Meeting Template and scroll down to the section called “Personalization.” Type what you would like reflected in the invitation in the text box for “Invite Notes” and it will appear on all of the invitations when you use this Meeting Template.