Hello @EdwardP102 ,
There are a few methods to accomplish what you are looking to do. You can add contacts on an individual basis into existing lists. If you're looking to add contacts to a list en masse, you can import a file of contacts (even if they're already in your account), and then simply add them to the appropriate list during the import process.
You can also use the paste-multiple method of uploading. This may be quicker than the file method, since reimporting a contact email will maintain all the info already associated with it in the reimport. Have your contact list open in one tab / window, have the add multiple import page open in another, then copy-paste email addresses as needed for whatever list you're trying to import to.
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