Count based on group membership

Hi everyone, I would like to get the number of countries that a given "Name" has Partners in. (See example sheet for an illustration)
I have the ID of a group in column A. Column B represents Names. Column C gives Info about the country of "Name".
In column D I would like to get the information how many unique countries a "Name"'s group members are from. This means, not only within a single group, but in total over all groups. This means, that every same Name has the same number of countries in column D.

Does anyone know how to do this?

Formula in words:
"sum of all unique countries in column C, if ID in column A matches any ID of Name in column B"

Thanks a lot in advance!