This script can be used to check if the email participants are related to a user and then check if that user is a JSM Agent, this would then allow to determine whether to add the request participant or add the user to a JSM Agent role first.
#set($jsdApiUtils = $jemhUtils.getJsdApiUtils())
#set($project = $projectManager.getProjectByCurrentKey('TASK'))
#set($user = $jemhUtils.getUserByEmail('admin@localhost'))
#if($jsdApiUtils.isAgentForProject($project, $user))
Yes
#else
No
#end