I previously used Calendly.com to let clients schedule appointments with me at their most convenient time. There are other similar utilities, such as vCita, but FindTime from Office 365 is included for free and works great within Outlook for either the Mac or Windows PC.
It works as an add-in to Outlook, so the icon above appears within your Outlook ribbon once installed. When an email arrives that suggests a meeting, then you click on the icon in the ribbon to bring up a selection of dates available on your calendar, as well as it displays available times for other users in your organization. External guests are grayed out, but that’s the beauty of the program – let others choose when they can attend.
Once a meeting time is established, the date is automatically entered into your Outlook calendar, which then syncs across all of your devices.
To install this add-in to Outlook, go to: