This exactly isn’t available in Strava. There are group challenges though  (see https://support.strava.com/hc/en-us/articles/360061360791-Group-Challenges) but all participants must be subscribers and calories burnt isn’t one of the possible goals.

Usually such corporate challenges are reported manually and underlaid with screenshots or sharings from the various fitness and health apps on smartphones, a complete automatic is unknown to me. Maybe others have a better idea though.

I don’t remember the full details, so I’m afraid this is more clue than answer, but I do know that there’s a tool in Google Chrome that allows you to download the details of a set of activities that you can then push into a spreadsheet for further processing.  As I recall the download comes in blocks and you may have to run the tool several times to get the full data set.  

API… folks from my workplace do exactly the same thing, annual challenge on burned calories. They've created a simple app using Strava’s API, it's crunching our data and creating individual/teams leaderboards, monthly graphs etc. 

It’s not trivial for the general public but if you have some developers, it's probably relatively easy: https://developers.strava.com