Round robin assignment of issues.

Ok, it's possible, but requires 1 additional (possible private) custom field of type user[*] to store projects assignees. You need to add additional field because of current restriction of customizable workflow system, please vote for JT-12689, JT-12701.

To assign each new issue to next assignee write the following workflow:

rule Round assignment 
 
when created != null { 
  var lastCreated = project.issues.first; 
  for each curIssue in project.issues { 
    if (curIssue.created > lastCreated.created) { 
      lastCreated = curIssue; 
    } 
  } 
   
  var timeToSetAssignee = false; 
  for each assignee in Assignees { 
    if (timeToSetAssignee) { 
      issue.Assignee = assignee; 
      break; 
    } 
    if (assignee == lastCreated.Assignee) { 
      timeToSetAssignee = true; 
    } 
  } 
}

Please look at the reference how to write own workflows.