Replace group_by option by subgroup

[tonyfujs]

Prework

• If there is already a relevant issue, whether open or closed, comment on the existing thread instead of posting a new issue.
• New features take time and effort to create, and they take even more effort to maintain. So if the purpose of the feature is to resolve a struggle you are encountering personally, please consider first posting a "question" or "other" issue so we can discuss your use case and search for existing solutions first.
• Format your code according to the tidyverse style guide.

Proposal

If applicable, write a minimal example in R code or pseudo-code to show input, usage, and desired output.

To help us read any code you include (optional) please try to follow the tidyverse style guide. The style_text() and style_file() functions from the styler package make it easier.