diff --git a/project_customer_access/models/res_users.py b/project_customer_access/models/res_users.py
index c9538964..9fe6b167 100644
--- a/project_customer_access/models/res_users.py
+++ b/project_customer_access/models/res_users.py
@@ -7,6 +7,41 @@
 class ResUsers(models.Model):
     _inherit = "res.users"
 
+    def _get_support_partner_domain(self):
+        """Domain of the partners visible by a customer user (used by the record
+        rule on res.partner): the partners of the users sharing the same
+        cross-connect client (i.e. the members of his company), the internal
+        users (internal users without cross-connect client, needed to display
+        the assignees / authors / followers of the tasks) and the customer
+        partners of the projects of this client, plus his commercial partner, the
+        companies and OdooBot.
+
+        The relations are traversed by SQL, so the domain stays up to date when
+        users/projects change. Users without a cross-connect client are not
+        restricted.
+        """
+        self.ensure_one()
+        client = self.cross_connect_client_id
+        if not client:
+            return [(1, "=", 1)]
+        return [
+            "|",
+            "|",
+            "|",
+            "|",
+            "|",
+            "|",
+            ("user_ids.cross_connect_client_id", "in", client.ids),
+            "&",
+            ("user_ids.groups_id", "in", [self.env.ref("base.group_user").id]),
+            ("user_ids.cross_connect_client_id", "=", False),
+            ("project_ids.cross_connect_client_id", "in", client.ids),
+            ("task_ids.project_id.cross_connect_client_id", "in", client.ids),
+            ("id", "=", self.commercial_partner_id.id),
+            ("id", "in", self.company_ids.partner_id.ids),
+            ("id", "=", self.env.ref("base.partner_root").id),
+        ]
+
     @api.depends("cross_connect_client_id")
     def _compute_display_name(self):
         return super()._compute_display_name()
diff --git a/project_customer_access/security/project_customer_access_security.xml b/project_customer_access/security/project_customer_access_security.xml
index 0db99ba9..242cd885 100644
--- a/project_customer_access/security/project_customer_access_security.xml
+++ b/project_customer_access/security/project_customer_access_security.xml
@@ -63,6 +63,20 @@
         <field name="groups" eval="[(4,ref('project_customer_access.group_budget'))]" />
     </record>
 
+    <record id="rule_project_customer_access_partners" model="ir.rule">
+        <field name="name">Project Customer Access: Partners</field>
+        <field name="model_id" ref="base.model_res_partner" />
+        <field name="domain_force">user._get_support_partner_domain()</field>
+        <field name="perm_read" eval="1" />
+        <field name="perm_write" eval="0" />
+        <field name="perm_create" eval="0" />
+        <field name="perm_unlink" eval="0" />
+        <field
+            name="groups"
+            eval="[(4,ref('project_customer_access.group_customer'))]"
+        />
+    </record>
+
     <record id="mail.mail_message_subtype_rule_public" model="ir.rule">
         <field
             name="groups"
diff --git a/project_customer_access/tests/test_project_customer_access.py b/project_customer_access/tests/test_project_customer_access.py
index 730d6ed9..46417d3d 100644
--- a/project_customer_access/tests/test_project_customer_access.py
+++ b/project_customer_access/tests/test_project_customer_access.py
@@ -133,3 +133,28 @@ def test_create_task_customer_no_project(self):
             "Uh-oh! Looks like you have stumbled upon some top-secret records.",
         ):
             f.save()
+
+    def test_visible_partners(self):
+        """Customer users see the members of their company, the users working on
+        their projects and the partners of these projects."""
+        colleague = new_test_user(
+            self.env,
+            login="test_colleague",
+            groups="project_customer_access.group_customer",
+        )
+        colleague.cross_connect_client_id = self.client
+        support_user = new_test_user(
+            self.env, login="test_support_user", groups="base.group_user"
+        )
+        self.task.user_ids = [Command.link(support_user.id)]
+        other_partner = self.env["res.partner"].create({"name": "Other Company"})
+
+        partners = self.env["res.partner"].with_user(self.customer).search([])
+        self.assertIn(self.customer.partner_id, partners)
+        self.assertIn(colleague.partner_id, partners)
+        self.assertIn(support_user.partner_id, partners)
+        self.assertNotIn(other_partner, partners)
+        with self.assertRaises(AccessError):
+            other_partner.with_user(self.customer).read(["name"])
+        # a user without cross-connect client is not restricted
+        self.assertIn(other_partner, self.env["res.partner"].search([]))