Need help adding new field to Joomla Registration Form

I have succesfully added a new field called 'company' to the standard regsitration form in Joomla. I have setup Joomlamailer signup so the check box appears on the reg form.

I'm following instructions from several sources explaining how to add fields using phpMyAdmin and I've reached the stage where I need to edit the user.php file normally found at libraries/joomla/database/table/user.php. I have edited the file correctly by adding the line var $company = null; after class JTableUser extends JTable {.

If I add myself as a new user and put something in the 'company' field, when I check the database for the new entry it is blank.

I suspect when using joomlamailer, the user.php file that needs editing is not the default one as I needed to edit a totally different file elsewhere in the instructions to make the new field appear on the form. Default file to edit is components/com_user/views/register/tmpl/default.php but with Joomlamailer installed I needed to edit /components/com_joomailermailchimpsignup/templates/register.php instead.

So my question is where is the file I need to edit to make registration form entries appear in the database?

Thanks for anyone who can answer this as I'm now partially bald!