Coworking/coworking_relation/views/relation_menu.xml

<?xml version="1.0" encoding="utf-8"?>

<odoo>

  <!-- Action to open Coworkers list -->
  <act_window id="relation_action" name="Relation"
    res_model="coworking.relation" view_mode="tree" />
  <!-- Menu item to open Event list -->
  <menuitem id="coworker_relation_menu" name="Relations" action="relation_action"
    parent="coworking_coworker.coworker_menu" sequence="2" />

</odoo>