Setting up a meeting is always a challenge with some people, they are always busy 
Sometime ago I wrote a post about FindTime, an add-on for Outlook or Outlook on the Web (OWA).
Now, Microsoft has integrated a similar feature in Exchange Online.
When you create a new meeting request when using Outlook on the Web (this is not [yet?] available with Outlook client) you can use the Make a pool feature
This will allow you to define different time slot before sending the meeting and then let the recipients to choose when they are available
When you enable the Make a pool feature, an additional ‘free/busy’ window is shown on the right side to let you select different time slot with one click
Once you send the meeting, the recipient will then be able to choose when they are available and In your calendar, the options are marked with an HOLD prefix
Recipients can choose their time slot by hitting the Choose time link which redirect them to the Outlook Scheduling site
The organizer will not get any notification (as it is usually the case when accepting/declining meeting) but the tracking status is updated with the answers.
You can also check the pool results from Outlook on the Web with the view pool results link
Using the same link you then will be able to finalize the meeting request
Once you hit the Send action to finalize the meeting request, all HOLD meeting in your calendar are removed and the final request meeting is sent.