When groupings are blank, it would take the default internal grouping, which is not desired
please define your function module and add it into this event,
this will be executed before BP save,
you can check but000-BU_GROUP, issue error
event BPNUM or BPNEX,
define own function module
Use badi BUPA_NUMBER_GROUP for making groupings to default. But this could only work for GUI mode BP transaction
Use badi BADI_CRM_BP_UIU_DEFAULTS to control attribute BP_GROUP, this will work for web ui in CRM2007.