Hi Raja,
I think Dhilipan has already explained how the Country Group is determined if field MUCGR is not set.
Table /SAPSLL/CTYGPA holds the assignment of Countries to Country Groups, with validity dates.
Table /SAPSLL/LRGCGP holds the assignment of Country Groups to Legal Regulations.
So, for a business partner in France, the system first finds entries in /SAPSLL/CTYGPA, then discards any that are not assigned to the Legal Regulation in /SAPSLL/LRGCGP. If any Country Groups remain, they are considered exactly as Dhilipan describes above.
Regards,
Dave