Importing group associations _only_ (Following question to this one)
Suppose I have an .xls or .csv file with associations of contact names to groups (or firstname, middlename, lastname to group).
I want to import these associations  - applied to existing contacts; I don't want to create new skeleton contacts based on these associations. Is this possible? If so, how?
The SA Contacts Lite application, which can import and export contacts more generally, can be used specifically for this purpose:

Download &amp; install SA Contacts Lite application; it is available on places where you don't need an account, e.g. via APKPure.com.
Remove all columns from your the file except for the names/name components and the group associations (i.e. remove all the columns you don't want to import)
Save the whittled-down file to the root of your phone's storage hierarchy
In SA Contacts Lite, click Import.
Choose SD Card as the location in which to search for the import file (or someplace else, if you put your file on Box/DropBox/Google Drive and have that configured in SA Contacts Lite; I didn't do this); press Next.
Select the .xls file in the dialog which comes up; press Next.

That's basically it. Your existing contacts will now also have the group associations.
Notes:

I've tried this with an Excel file: .xls, not .xlsx. Not sure if it also works with .csv files.