diff --git a/inc/Users.php b/inc/Users.php
new file mode 100644
index 0000000..7679e04
--- /dev/null
+++ b/inc/Users.php
@@ -0,0 +1,51 @@
+<?php
+namespace DAFME\Covid;
+
+class Users {
+  private static function getUserId($sub) {
+    global $con;
+    $query = $con->prepare('SELECT id FROM users WHERE sub = ?');
+    if (!$query->execute([$sub]))
+      return false;
+
+    if ($query->rowCount() < 1)
+      return false;
+
+    $row = $query->fetch();
+    return $row['id'] ?? false;
+  }
+
+  public static function add($sub, $email) {
+    global $con;
+    $query = $con->prepare('INSERT INTO users (sub, email) VALUES (?, ?)');
+    if (!$query->execute([$sub, $email]))
+      return false;
+
+    return $con->lastInsertId();
+  }
+
+  public static function signIn($sub, $email) {
+    global $_SESSION;
+
+    $userId = self::getUserId($sub);
+
+    if ($userId === false)
+      $userId = self::add($sub, $email);
+
+    if ($userId === false)
+      return false;
+
+    $_SESSION['userId'] = $userId;
+    return true;
+  }
+
+  public static function signOut() {
+    global $_SESSION;
+    unset($_SESSION['userId']);
+  }
+
+  public static function isSignedIn() {
+    global $_SESSION;
+    return isset($_SESSION['userId']);
+  }
+}