since a subreport is basically a report, write a report that will create a string (let's say) that is the order for each group. Then you can set a shared variable to the string in the subreport.
when that is working like you want, in the main report, insert a subreport, and use the report you have already written. I would create another section the group header and move it on top of the existing section and place the subreport in this section...this way the subreport will execute first. Now, for the 'original' group header, you can place a suppression formula using the same shared variable as in the subreport and you can compare the outputs.
in simple terms the suppression formula would look like:
shared stringvar a; //in the subreport it would also be named a
a<>"xyz"
or
a<>"yzx"
some notes about the subreport, you will want to suppress the display output, and in the main report in the Section Expert, you will want to set the open to suppress blank rows on, this way the subreport will run, but will not display...you can display everything until you get everything the report working. You will also need to link the subreport to the main report so that it only displays the values for the group you are in. You will select 'Change subreport links' and match the field values in the main report to the parameter values in the subreport.
I know that there is a lot of info, and most of it is vague, but it should cover the points you need to find more concrete examples / items to look up in CR help.
HTH