You probably have contacts who work with an assistant to schedule their meetings and, when your guest has an assistant, he/she will most likely be the one exchanging with Julie.
In this case:
- Copy Julie and the assistant on the email, like you do otherwise
- Indicate that the person on copy is your guest’s assistant and that in order to coordinate a meeting, Julie must exchange with them to organize a meeting according to your specifications.
Ex: « Julie, please coordinate with Antoine, Mr. Durand’s assistant for a lunch this month.”
Once the date is found, Julie will ask for the actual email address of your guest to send an event invite with all necessary information attached.