'Comment' tab in ticket not availlable for all

'Comment' tab in ticket not availlable for all

Dear support      
can you please help me figure out how I can add the tab 'Comment' to the departments?
I'm not sure if this depends from dept or agent or what, but I've a situation in which 1 out of 5 dept can use comment to collaborate with customers and the others not.
I would like to extend the comment functionality  to everybody. 
Thanks for your help
BR
Diana