How to make a taxonomy term display a field
Create a view which shows surname of person given a username. Use https://drupal.org/project/viewfield and add it as field in the taxonomy and it should show surname.

By default linked users are not listed under taxonomy terms (read here : Assigning User&#39;s In Taxonomy Terms) and there is another useful post: https://stackoverflow.com/questions/11524357/how-to-link-a-user-with-taxonomy-terms