I have a list of contacts (names and email addresses). I know that all of these contacts are in the Civi database, but I want to add a tag or a group to all of them, and i have the list in a CSV file. What I should be able to do is simply import the contacts, setting 'Update' or perhaps 'Fill' as my duplicate matching option, and set the tag and/or group to be set in the import settings. Just tried it - several times, using various different options, and it failed every time, reporting that 0 records had been tagged (or added to the group). This is with Civi 4.7.19.
Is there a specific set of options that I need to know about to get this to work? This post (How to tag a contact when importing possible duplicates) suggests that this should just work. I followed that example and it failed.