Not unless the group total is already calculated...like in a stored proc. CR has to figure out what the group total is, and it does that later in the process (when displaying the report) than grouping. Grouping is off of data that exists in the table, pretty much directly. You can group on a field, several fields in combination, but not the average of a group as that does not exist in the data table.
If you get your data from a stored proc, have it calculate the values that you need and return them as a column...if you have the total of the group, then you could sort on the calculated value as both parts of the calculation that are needed are available to CR at the time that the data is read.
Hopefully, this is clear enough