adding a mailing list from a spreadsheet

From Contacts Help, do an text file export/import process:

Import one or more contacts (from a file other than a vCard)
You can import contact information that you saved or exported from other apps. The files can be in LDAP Interchange Format (LDIF) or text format (tab-delimited or comma-separated value (CSV)).

If you’re importing a tab-delimited or CSV file, verify it using a text editor such as TextEdit.
Remove any line breaks within a contact’s information. Line breaks can cause the import to fail.

Make sure that fields in a tab-delimited file are separated by a tab, instead of by another character. Don’t include spaces before or after tabs.

Make sure that fields in a CSV file are separated by a comma, instead of by another character. Don’t include spaces before or after commas.

Make sure all addresses have the same number of fields. Add empty fields as needed.

In Contacts, choose File > Import, select the file, change the encoding if necessary, and then click Open.
If you’re importing a text file, review the field labels. You can change field labels, or ignore or not import fields.
If the first card contains headers, make sure the headers are correctly labeled or marked “Do not import.” Any changes you make to this card are made in all of the cards in the file. To not import the headers card, select the “Ignore first card” checkbox.

To change a label, click the arrows next to the label and choose a new label. If you don’t want to import a field, choose “Do not import.”