I didn't completely understand what you mean by casual user. However each user needs a demand management license and the user must be memeber of the group ppm users to get the required access grants. The user must be configured in the security settings of the step as well. It can be a security group, username, user_id, token etc.
Hi Sanjay. You need this: if you have any field for example "Casual_user". Thi field have tpken "Casual_user". in WF step you need set security user defined token as "REQ.P.Casual_user". In in this fiel set 1 or more users then they can edit and press button in this step of WF. Good Luck