This script will allow you to 'round robin' assign issues to users in a certain project role. Round-rogin assigning means automatically assigning a different user each time a tranisition occurs. Add this as a ScriptRunner Post Function to automatically assign the next user in the list. You'll need to add in the project role and key that you want to use where specified in the script. If you want assignment to occur at creation, set this up as a listener on the 'Issue Created' event.