A couple of questions:
- are these tables in the same data source?
- can a Customer be a part of more than one Buying Group?
- can you please provide what you expect for a result from this example data?
If they are tables in the same data source, and each Customer is only in a single Buying Group, then you can use the Multiple Tables option when you create or edit your connection.
Otherwise you will need to use a data blend and calculated fields.
With that information, an answer can be provided.