Best way to combine data within a group

How can I alter my expression to show a merged cell for the locations located in group A and B (see snapshot below)

Basically the "Limit" and RIIR is based on the sites combined, Unlike the sites located in group C which are measured individually.

What is the best way to do this?  Here is the expression that I am currently using to Calculate the RIIR based on the subgroup which is Sites, it's Parent Group is called Location_2.

RIIR Field =

=Fields!YTD_OSHA_RECORDABLES.Value*200000/Fields!YTD_EMPLOYEE_HOURS.Value

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Basically I want to keep everything but the Limit and RIIR individual for each site...I just want to combine the RIIR and LIMIT for the groups a and b.

Thanks,

M