Add contacts to group using CSV with contact e-mails
You have to select an appropiate Dedupe Rule so that the importer can detect by which criteria you want to treat an existing contact and a imported contact as the same.

Using the predefined Email - Unsupervised Rule should work for this purpose.



If you chose the same location as is defined for the contact email it will just update the existing contact.

However, the option to select the primary e-mail does not seem to be working as expected. It will create a new adress without setting a proper location. 



You may want to report this as a bug.