How to create a simple map with one-to-many relations?

Dear All,
I hope I have a simple problem, with which you can help me out.

I am trying to make a map of firms of certain branch of industry and there is a situation that one polygon (e.g. municipality) has connections to more than one element (firm). So I can not make a single join, because it can handle only one-to-one relations.
I have a polygon layer of the municipalities of the region and a xls table, where each row is a separate firm to join to a certain polygon.

There is a long and complicated way, as to create as many tables as many elements I have to join to a certain polygon, and make "multiple" single joins, but I hope there will be a simplier and smoother solution.

Thank you in advance!
Charles