Once in the grid, first we prepare the grouping, in this case we will try with a static one
Then we proceed to configure the fields that are needed
Once the configuration is obtained, go to the summary configuration
In the fields section, we will have the following view and select edit:
What the following screen will bring us and select the Multiselect box option:
Finally, we also configure the Lookup options section
Clic in button run aplication.