I am not sure which version of CR you are using. I believe CR2008 now can create summaries of groups. However, if you are using an older version you will probably have to create a manual running total. Essentially you create a shared variable that runs during the whileprinting pass. You do this by creating 3 formulas and placing them in appropriate places in your report design.
You will create the first formula and probably place it in the report header. This sets the shared var to an initial startnig value.
You create a second formula that creates and uses the same shared variable from formula 1 again set to run during the whileprinting pass. You then increment the shared variable typically by adding the summary amount. This would probably be placed in the group footer.
Finally your do this all one more time but place this in the report footer and simply declare the shared variable and return it for printing.
All of this has been discussed a number of times on the forum, and is in fact somewhat buried yet found in the CR Help file.
Hope this helps.
Regards,
John W.