There's no reason you cannot share parameters. Many people make the mistake of creating the parameters again in the subreports - this leads to confusion!
The parameters should only be defined in the main report.
Using the subreport links tab you can select the parameters from the main report and they will be passed to the subreport (crystal will create new parameters in the subreport automatically prefixed with '?Pm-').
As Madawc points out - the subreport links tab will only allow you to link the parameters to fields in the subreport of the same data type as the parameter. This may cause confusion if there are no fields in the subreport of this type (e.g. passing in a date parameter when only date/time fields exist).
In these cases, create a formula field in the subreport of the type needed
e.g. @test_date
date(2004,01,01)
Then retry linking the date parameter to the subreport's @test_date formula.
The last step, again as Madawc points out, is to check the select expert in the subreport. It defaults to
@test_date = ?Pm-?passed_in_parameter
You can now change this to whatever you intended to do with the passed-in parameter. In fact, on some occasions, you might not want to use the parameter in the selection at all.
Hope this helps
Steve Phillips, Crystal Trainer/Consultant