Big change is coming to the Subscribers section and the adding of subscribers to Lists will be far more convenient.
Until this update, you can do it the same way as you update subscribers by import. In short this means
- You have to prepare a file (CSV or TXT) with all the subscribers you would like to add to the List. Only the column with their email addresses is necessary, there is no need for other subscribers' details. So file with just a list of email addresses is perfectly fine.
Now import these subscribers as usually, just
- specify the List in which they will be added (1), you can choose more of them,
- tick Update actual contact (2) so only the information about Lists will be added to each contact and it will not be overwritten.
Now finish the importing process. If the subscribers were already present in your audience, they were not imported again, only the attribute that says they are part of a List was added to their profiles.