In Calendar preferences you can define who can invite you to meetings. The following are the invite options:
Allow both internal and external users to invite me to meetings. This is the default. Anyone can send you an meeting invitation.
Allow only internal users to invite me to meetings. Only internal users can send you an meeting invitation. Meeting invitations from external users are rejected.
Don't let anyone invite me to meetings. You can send meeting invitations and keep your own appointment schedule, but you do not receive any invitation messages from other users.
Allow only the following internal users to invite me to meetings. You list the internal users who can send you meeting invitations. Only those internal user addresses listed have permission to send you a meeting invitation. Meeting invitations from all other users are rejected.
If you limit who can invite you to meetings, you may want to select to send an auto-reply message to those users denied permission to let them know they do not have permission to invite you. Otherwise the person sending an invitation does not know that they do not have permission to send you an invitation.