I am not sure what you mean by sharing and group...
A shared variable is a single value (or an array) and you can't really group by the shared variable, as there is only 1 per report, and doesn't exist at the time that the data is being read from the database
You can use a shared variable to filter data (at least I think you can...never tried)
As far as naming conventions, I've never had a large enough report to need a convention, so to start, probably any convention will work
HTH