If you go to My Stuff > DM Threads - I am the seller > click on the # of sales for an item > Bulk add users > List of emails
It would be really nice to be able to list forums users here by name since many users do not list their email address. For example, I wanted to grant Mike access to PEP BB Calculator for Google Sheets, but entering "Mike Haasl" ends up failing and I don't know Mike's email address to enter since it isn't listed in his public profile.
I suppose this could be a spam prevention 'feature' to keep sellers from spamming out free stuff, but if not, please consider it a feature request. :-)