invites: Enable adding users to user groups during invitations.

This commit allows users to be assigned to custom groups when
inviting them to join Zulip, similar to how channels are handled.
The implementation follows a similar pattern for adding pills,
ensuring consistency, as user groups and channels are parallel
in nature.

Fixes #24365.

zerver/tests/test_signup.py

@@ -1039,7 +1039,7 @@ class LoginTest(ZulipTestCase):
         # to sending messages, such as getting the welcome bot, looking up
         # the alert words for a realm, etc.
         with (
-            self.assert_database_query_count(94),
+            self.assert_database_query_count(95),
             self.assert_memcached_count(14),
             self.captureOnCommitCallbacks(execute=True),
         ):