To create folder, select "New" from "Folder" menu, then enter its name.
To rename a folder, select it, use "Rename" command from "Folder" menu and enter new name.
To remove folder, select it and use "Delete" command from "Folder" menu. Note that there's no "undo" option.
Having any folder selected (focus, or keyboard control, must be set on folder list), using "New message" from "Address" menu allows you to create message for all contacts in that folder.
Addresses:
To add new contact (address), use "New" command from "Address" menu and enter person/contact data. Cick "OK" when done.
To modify contact data, click it twice or use "Modify" command from "Address" menu.
To remove addresses, select them and use "Delete" command from "Addresses" menu. Note that this is irreversible.
To move address(es) into a different folder, select them and then drag into desired folder. Hold down "Shift" key to copy them instead of moving.
Having selected one or more contacts, you can start composing message for them by using "New message" from "Address" menu. Note that folder contents panel should be focused (have keyboard control), otherwise new message will be addressed to all contacts in active folder.
Address editor doesn't require any special comments, except for possibility of omitting birthday entry. To leave person's birthday empty, remove checkmark next to the date in birthday field.