How to configure a role so that both created_by and assigned_to users can edit the record

How to configure a role so created user and assigned user both can edit record. 

Ex: user 1 created a case and assigned to user 2. if user1 wants to add or change some field value he can not access edit record because record assigned to user2. so how to overcome this problem how to setup a role that both can edit record.hatsShahab KhanFrancesca ShiekhAlex Nassi

needed help as soon as possible..