if you just need to schedule something in the calendar, then take a look at CalDAV.
If enabled and your user has sufficient permissions, you can access your own calendar like this:
Access any other users calendar, if your user has sufficient perrmission to do so:
In order to schedule a new event, you need to generate an ICS file and then submit it to groupdav.php using PUT method, for example using curl.
Here is an example in PHP:
For further details may be translate this detailed german how to:
CalDAV How To
I found this topic because I was looking for an egroupware (REST) API myself, because the CalDAV interface doesn’t seem like a good way to access a users meeting list for the current day. In order to achieve that, one has to the query string “?download” to the groupdav.php call, but this returns all appointments in the near feature.
Hope that helps.