Allowing other users to modify your Tasks
(Agenda View | Tools | Options | Access Rights | Designate)
Control who can modify your Tasks
by setting viewing rights.
To grant modifying rights for your Tasks:
Choose what access level of your Tasks you want each user to view, or set
a user's viewing rights to Use Defaults. If you grant users the
right to view a type of Task (for example, Confidential), they will have
full viewing rights for all Tasks of that type.