To set up meetings and integrate your calendar, navigate in your HubSpot Sales account to Settings. On the left, select Sales > Meetings.
Next, click Connect your calendar, then click Connect your Google Calendar or Connect your Office 365 calendar. To check if your email provider is compatible with meetings, refer to this article.
Finally, when prompted, click Allow to give the meetings tool access to your calendar.
Once you have set up the integration, the meetings tool will automatically prevent clients from booking meetings during any events that are set to show as Busy on your integrated calendar. This helps prevent double-bookings and you never need to bother letting a prospect know when you are free.
With this two-way integration, meetings created through the Schedule tab on a contact record in the CRM will send actual calendar event invitations to the contact. Without your calendar connected, it will strictly just create a meeting engagement on the record.
This two-way integration is not available for Office 365.