I have tried presenting user with a prompt and asking them to select the field to group on. Then I tried to use that parameter as a group, but parameters aren't available for grouping, so I used the parameter in a formula (stringvar groupon:={?groupParam} and then used that formula to group on.
It allows me to do it, but when I run the report I get a "Group specified on a non-recurring field. Details;@GroupOn