1. Create a selection of the members you want to add the note to by 'Searching' for them
In this case I used Search > Personal to locate all of my constituents in Texas because I sent them a mailing.
2. Once the correct members are selected, from the 'Members' menu select 'Tag Selected'
3. On the 'ADD CONTACT' side you can add a 'Contact by:', 'Contact type:', 'Contact method:', and/or a contact 'Note', just fill out what you want and press 'Add Contact Record'
It's just like adding a contact to an individual record but this will add an individual contact note to all selected records.
Update: Contact notes can now be deleted in batches as well.
Click here for more information on deleting multiple contact notes.