the information needs to be the data already, ie it has to be something that is easily tested, like {table.dateField} > someDate.
with that said, create a formula that sets up the group that you want, like:
if @group = 1 then
{table.field1}
else
{table.field2}
The only proviso is that the IF statement has to return the same type of value both branches. So you can't return a date and a number, or a string and a number like:
if x then
1
else
"a"
Once you have the formula, create a group and use the formula to group on instead of field in a table.
HTH