You have two option. First, yes you should be writing the choice to a user specific column so it doesn’t interfere with other users making choices.
First option is as @Robert_Petitto suggested. Add an inline list that points to the member sheet, then create a filter for that list that filters the members by the screen value of the choice.
Second option is to create a relation column that links the user choice to the member sheet. Then add an inline list that points to that relation.
That’s assuming that you want to display the choice and members in the same tab. If you instead want to make your choice in the region tab and have the results updated in the member tab, then you need to create a single value column in the member sheet that gets the selection, then filter the member tab list to only show those members where the member region matches the single value region.