diff --git a/batch/batch/exceptions.py b/batch/batch/exceptions.py
index 43a52451655..4ce2ff78aab 100644
--- a/batch/batch/exceptions.py
+++ b/batch/batch/exceptions.py
@@ -37,6 +37,11 @@ def __init__(self, batch_id):
         super().__init__(f'Batch {batch_id} does not exist.', 'error')
 
 
+class NonExistentUserError(BatchUserError):
+    def __init__(self, user):
+        super().__init__(f'User {user} does not exist.', 'error')
+
+
 class OpenBatchError(BatchUserError):
     def __init__(self, batch_id):
         super().__init__(f'Batch {batch_id} is open.', 'error')
diff --git a/batch/batch/front_end/front_end.py b/batch/batch/front_end/front_end.py
index 2f6b0a66ad9..a9740788ef4 100644
--- a/batch/batch/front_end/front_end.py
+++ b/batch/batch/front_end/front_end.py
@@ -41,6 +41,7 @@
     setup_aiohttp_session,
     transaction,
 )
+from gear.auth import get_session_id, impersonate_user
 from gear.clients import get_cloud_async_fs
 from gear.database import CallError
 from gear.profiling import install_profiler_if_requested
@@ -81,6 +82,7 @@
     ClosedBillingProjectError,
     InvalidBillingLimitError,
     NonExistentBillingProjectError,
+    NonExistentUserError,
     QueryError,
 )
 from ..file_store import FileStore
@@ -2556,7 +2558,17 @@ async def api_get_billing_projects_remove_user(request: web.Request) -> web.Resp
     return json_response({'billing_project': billing_project, 'user': user})
 
 
-async def _add_user_to_billing_project(db, billing_project, user):
+async def _add_user_to_billing_project(request: web.Request, db: Database, billing_project: str, user: str):
+    try:
+        session_id = await get_session_id(request)
+        assert session_id is not None
+        url = deploy_config.url('auth', f'/api/v1alpha/users/{user}')
+        await impersonate_user(session_id, request.app['client_session'], url)
+    except aiohttp.ClientResponseError as e:
+        if e.status == 404:
+            raise NonExistentUserError(user) from e
+        raise
+
     @transaction(db)
     async def insert(tx):
         # we want to be case-insensitive here to avoid duplicates with existing records
@@ -2588,6 +2600,7 @@ async def insert(tx):
             raise BatchOperationAlreadyCompletedError(
                 f'User {user} is already member of billing project {billing_project}.', 'info'
             )
+
         await tx.execute_insertone(
             '''
 INSERT INTO billing_project_users(billing_project, user, user_cs)
@@ -2605,13 +2618,13 @@ async def insert(tx):
 async def post_billing_projects_add_user(request: web.Request, _) -> NoReturn:
     db: Database = request.app['db']
     post = await request.post()
-    user = post['user']
+    user = str(post['user'])
     billing_project = request.match_info['billing_project']
 
     session = await aiohttp_session.get_session(request)
 
     try:
-        await _handle_ui_error(session, _add_user_to_billing_project, db, billing_project, user)
+        await _handle_ui_error(session, _add_user_to_billing_project, request, db, billing_project, user)
         set_message(session, f'Added user {user} to billing project {billing_project}.', 'info')  # type: ignore
     finally:
         raise web.HTTPFound(deploy_config.external_url('batch', '/billing_projects'))  # pylint: disable=lost-exception
@@ -2624,7 +2637,7 @@ async def api_billing_projects_add_user(request: web.Request) -> web.Response:
     user = request.match_info['user']
     billing_project = request.match_info['billing_project']
 
-    await _handle_api_error(_add_user_to_billing_project, db, billing_project, user)
+    await _handle_api_error(_add_user_to_billing_project, request, db, billing_project, user)
     return json_response({'billing_project': billing_project, 'user': user})
 
 
diff --git a/batch/test/test_accounts.py b/batch/test/test_accounts.py
index 941f2c64b93..d08fd5702f7 100644
--- a/batch/test/test_accounts.py
+++ b/batch/test/test_accounts.py
@@ -8,7 +8,7 @@
 import pytest
 
 from hailtop import httpx
-from hailtop.auth import session_id_encode_to_str
+from hailtop.auth import async_get_user, session_id_encode_to_str
 from hailtop.batch_client.aioclient import Batch, BatchClient
 from hailtop.utils import secret_alnum_string
 from hailtop.utils.rich_progress_bar import BatchProgressBar
@@ -279,6 +279,15 @@ async def test_add_and_delete_user(dev_client: BatchClient, new_billing_project:
     assert r['user'] not in bp['users']
 
 
+async def test_error_adding_nonexistent_user(dev_client: BatchClient, new_billing_project: str):
+    with pytest.raises(httpx.ClientResponseError) as e_info:
+        with pytest.raises(httpx.ClientResponseError) as e_user:
+            await async_get_user('foobar')
+        assert e_user.value.status == 401
+        await dev_client.add_user('foobar', new_billing_project)
+    assert e_info.value.status == 403
+
+
 async def test_edit_billing_limit_dev(dev_client: BatchClient, new_billing_project: str):
     project = new_billing_project
     r = await dev_client.add_user('test', project)
diff --git a/gear/gear/auth.py b/gear/gear/auth.py
index cae28c43751..0d91e51b6b0 100644
--- a/gear/gear/auth.py
+++ b/gear/gear/auth.py
@@ -120,10 +120,14 @@ async def _fetch_userdata_from_auth_service(session_id_and_session: Tuple[str, h
 
 
 async def impersonate_user_and_get_info(session_id: str, client_session: httpx.ClientSession):
-    headers = {'Authorization': f'Bearer {session_id}'}
     userinfo_url = deploy_config.url('auth', '/api/v1alpha/userinfo')
+    return await impersonate_user(session_id, client_session, userinfo_url)
+
+
+async def impersonate_user(session_id: str, client_session: httpx.ClientSession, url: str):
+    headers = {'Authorization': f'Bearer {session_id}'}
     try:
-        return await retry_transient_errors(client_session.get_read_json, userinfo_url, headers=headers)
+        return await retry_transient_errors(client_session.get_read_json, url, headers=headers)
     except aiohttp.ClientResponseError as err:
         if err.status == 401:
             return None