FreeBusy for Google Calendar lets you pick the best time to meet by showing you when all attendees are available, whether they are inside or outside your company, and irrespective of which calendar system they use.
FreeBusy… · More for Gmail add a Schedule button next to Compose so you can schedule meetings right from your inbox. Click Schedule, enter meeting details and select participants. FreeBusy will then show you a unified view of everyone's availability so you can pick a time that works for everyone or create a poll with multiple suggestions.
FreeBusy is also the easiest way to share availability across all your calendars so contacts can book meetings with you. You can link work, school, or personal calendars from any provider (Google, Outlook, Exchange, Office 365, iCloud, Yahoo, and many others).