client_id, client_secret: Application OAuth client ID and client Secret. These are generally not required for rtweet_user() since the defaults will use the built-in rtweet app.
app: Name of the client, it helps if you make it match with the name of your app. On the Twitter app the Callback URI must be http://127.0.0.1:1410/
(the trailing / must be included).
scopes: Default scopes allowed for users using this client. Leave NULL to allow everything or choose yours with set_scopes().