I don't know of a way to synchronize an .xls file with a Thunderbird address book. I did a quick search for a Thunderbird add-on to do this, and a Google search, and came up with nothing.

You can export your spreadsheet to a .csv file, then import it into Thunderbird.

Save the list from spreadsheet to a .csv file, with the first row being the column names. See here for a list of field names that Thunderbird can import and their names: http://kb.mozillazine.org/Import_address_list_from_text_file

In Thunderbird, open the Address Book and click on "Tools" in the menu, then "Import".

Select "Address Books" and click "Next". Select "Text file (LDIF, .tab, .csv, .txt)" and click "Next".

In the "Select address book file" window, just above the "Open" and "Cancel" buttons on the bottom right, change the file type to "Comma Separated (*.csv)". Then navigate to where you saved the .csv file you exported from Excel, choose it, and click "Open".

Now you will get an "Import Address Book" window where you will need to match up Thunderbird address book fields with the fields that are in your .csv file. Check mark the fields you want, and move them up or down to match the fields in your .csv file. When done matching, click "OK".

That will create a new address book with the name of the .csv file.

Delete the old address book containing your Member's contact info.

Yikes, this is a bulky process.

Batch updates of hundreds of names should be a standard feature.

I've marked it as a solution, since it works.