To allow your collaborators to add timeline items:
Go to the Collaborate tab at the top of the desired timeline.
On the right, click the name of the collaborator to whom you wish to grant these rights.
Just to the left of the list of collaborator names, you'll see an option "Can add timeline items: yes no". Select Yes.
Now when your collaborator visits their timeline panel, they will see a button next to the Help icon called "+Add". They'll be able to add items, hair and makeup schedules, timetables, and text sections. You will be notified as usual of any changes they make to the timeline.
You're all set!
The default is still set to not allow your collaborators to add items, so if you do not wish them to have these rights, you do not have to do anything :)
For visual learners, watch this in depth tutorial on the collaboration features: