Got into this Cortana Calendar assistant service and it's pretty cool.

 

You invite people and copy Cortana asking the service to find a time that you both can meet. Cortana looks into your calendar and offer some options to everyone invited. Depending on the responses Cortana will then put the meeting in your calendar and send an invite to everyone else.

 

(Yes, in this case I invited myself from different Office 365 accounts. The service works with Office 365, Outlook.com and Google Calendar. The invitees don't need to be using this service or these calendars).