Report Groups are found in System Settings, on the AR tab.
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You can have up to 12 different report groups. The names are free text.
Once you have created the report groups, you will need to modify each customer to set their report group. When customers are created, the report group is automatically set to 1.
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NOTE - not all reports use Report Groups. Currently there are only a couple of reports. The specific documentation on each report will indicate whether report groups are used.