BI Publisher RTF Template AR Invoice : Grouping rows

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I am required to create an AR Invoice report, where services are needed to be grouped based on what kind of service they are within a report. To be more clear,  for example service A and Service B should be together with a sub total, then Service X and Service Y together with a sub total and last the grand total of all the sub total. There are no fields, except the service it self with which I have to group. Need some guidance as to how I should proceed or what type of condition I should use.

Any help would be greatly appreciated.