Development is still working on enhancements to Buyer Group email list management features, so unfortunately at the moment, until those upgrades are finished, there is no way to manage or view Buyer Group lists from Seller Admin.
You can at least go to Seller Admin > View Transaction Log, select a date range and/or specific product, then click View Log to see only transactions for the selected dates/product, then Download Log to a spreadsheet file where you could e.g. select the Buyer Emails column to copy and paste however you wish. If you import that list into, say, Aweber, then I think they start out by sending each email an opt-in message to confirm their wish to subscribe.
