1. No you can't do groups on cubes. This is frequently requested feature, so I'll let product management know that there is another request.
2. No, you can't do a calculated field on a dimension on a cube. (You can make one on measures.)
There are a couple alternatives:
A. You can create calculated members -- see screen shot for where it is in the new UI. This means you'll have to use MDX, which can be tricky.
B. You can use the data integration features of 6.0, to use a grouping based on a relational source. Here's how I would do it.
1. Create a crosstab with the dimension and some other measure.
2. Export the crosstab to Excel.
3. Rename the heading of the measure column to the name of your group.
4. Change the numbers to group member names.
5. Save the Excel file.
6. Connect to the Excel file.
7. Use it as a secondary data source.
I hope this helps.
I think B is the best solution for me - thanks for the great advice
I tried this, and while Tableau 6 does establish the link between the similar dimensions, when I try to use the "group" field from one table against a measure from the other table (eg revenue), the groups are represented as a single * next to a single giant bar of revenue. What am I missing?