Symptoms
Users in SharePoint groups
Diagnoses
Updating users within Sharepoint that are already within a workflow, the new users don't get the email notifications in question. How do you get around this ?
Resolution
You can use the Identity Cache tool to set the cache timeout to what you would like it to be.
http://help.k2.com/onlinehelp/k2blackpearl/icg/4.6.10/webframe.html_tweaking_identity_cache_performance_for_the_k2_server.html
http://help.k2.com/onlinehelp/k2blackpearl/icg/4.6.4/webframe.html_refresh_usermanager_cache.html