diff --git a/src/applications/conpherence/editor/ConpherenceEditor.php b/src/applications/conpherence/editor/ConpherenceEditor.php
index e0a38ba8f5..75776cadfe 100644
--- a/src/applications/conpherence/editor/ConpherenceEditor.php
+++ b/src/applications/conpherence/editor/ConpherenceEditor.php
@@ -478,4 +478,43 @@ final class ConpherenceEditor extends PhabricatorApplicationTransactionEditor {
     );
   }
 
+  protected function validateTransaction(
+    PhabricatorLiskDAO $object,
+    $type,
+    array $xactions) {
+
+    $errors = parent::validateTransaction($object, $type, $xactions);
+
+    switch ($type) {
+      case ConpherenceTransactionType::TYPE_PARTICIPANTS:
+        foreach ($xactions as $xaction) {
+          $phids = $this->getPHIDTransactionNewValue(
+            $xaction,
+            $object->getParticipantPHIDs());
+
+          if (!$phids) {
+            continue;
+          }
+
+          $users = id(new PhabricatorPeopleQuery())
+            ->setViewer($this->requireActor())
+            ->withPHIDs($phids)
+            ->execute();
+          $users = mpull($users, null, 'getPHID');
+          foreach ($phids as $phid) {
+            if (isset($users[$phid])) {
+              continue;
+            }
+            $errors[] = new PhabricatorApplicationTransactionValidationError(
+              $type,
+              pht('Invalid'),
+              pht('New thread member "%s" is not a valid user.', $phid),
+              $xaction);
+          }
+        }
+        break;
+    }
+
+    return $errors;
+  }
 }
diff --git a/src/applications/transactions/editor/PhabricatorApplicationTransactionEditor.php b/src/applications/transactions/editor/PhabricatorApplicationTransactionEditor.php
index b80c75e3e2..307418c3a7 100644
--- a/src/applications/transactions/editor/PhabricatorApplicationTransactionEditor.php
+++ b/src/applications/transactions/editor/PhabricatorApplicationTransactionEditor.php
@@ -1397,9 +1397,14 @@ abstract class PhabricatorApplicationTransactionEditor
   }
 
   protected function getPHIDTransactionNewValue(
-    PhabricatorApplicationTransaction $xaction) {
+    PhabricatorApplicationTransaction $xaction,
+    $old = null) {
 
-    $old = array_fuse($xaction->getOldValue());
+    if ($old) {
+      $old = array_fuse($old);
+    } else {
+      $old = array_fuse($xaction->getOldValue());
+    }
 
     $new = $xaction->getNewValue();
     $new_add = idx($new, '+', array());