What permission is required to allow users to add work items to a plan in RTC?

 We are using RTC 6.0.3 and are seeing an issue that I didn't think we had previously...  For most of our project areas, we limit the permissions to Modify a Plan to the lead roles.  In the past, I thought that our general team members could create a new work item in a plan view and click the Save button to save the work items and have them show up in the plan.  I thought that just having permission to create a work item and set the filed against and planned for fields were enough to add a new work item to a plan.   I am now having issues where one of my users gets an error when he creates a new work item and hits Save.  It tells him he needs the Modify a Plan permission.