Azure: DocumentDB Mongo $group is not supported
There are two ways you can try
(i) You could try to follow documentdb-lumenize wrote by Larry Maccherone to provide Aggregations (Group-by, Pivot-table, and N-dimensional Cube) and Time Series Transformations as Stored Procedures in DocumentDB
(ii)Recently this feature is supproted by cosmosdb , you can enable this by Preview features pane and turn on MongoDB aggregation pipleline. Follow the following link,
For anyone trying to solve this problem in 2018+, this feature is now supported by DocumentDB, but is turned off by default. You must go into the 'Preview Features' pane in Azure and turn on 'MongoDB Aggregation Pipeline' to have access to $group.
For anyone interested in learning more, their announcement is below.