How to add Group By to a transformer?

Yes, you might be in luck. There is a way to expose the Group By parameter on a transformer. However, please note, there might be a reason certain transformers don’t support Group By, out of the box (e.g. transformers that process each feature individually won't work with a Group By parameter). So please verify the outcome of the translation carefully, and make sure it is producing the expected result, when you use this hack. Here it goes:

1. Right click on the transformer in question and Create Custom Transformer from it.

    

   (more detail on Custom Transformer )
2. In the Custom transformer tab, find “Parallel Process By” under Transformer Parameters>Advanced. Right Click and Create User Parameter…

    

   You may consider changing the parameter Prompt to “Group By”

    

   (This part of the advanced training can be helpful here. Although, no need to enable Parallel Processing, if you only want the Group By feature.)
3. Now the Group by parameter is available to be specified in the Main tab, on the transformer in question.

*If you are using a FME Version earlier then 2019.0, there is a caveat about this hack:

To make Group by work this way, Feature Caching needs to be turned off. (this issue has been fixed and available in 2019.0 beta)