In Gradebook Classic, when you "Assign TA" in Section Info, that teaching assistant is able to view and grade for their section. For example instructions below:
Assignments and T&Q also function in this way. So far, I have not been able to get the this to work in the new Gradebook. As a teaching assistant assigned as a TA to a section, I'm able to view/edit grades in Gradebook Classic. For the new Gradebook, in the same role on the same site, I receive the message: "You do not have permission to view the gradebook. Please contact your instructor."
Note: This is also been logged in Github (https://github.com/sakaiproject/sakai/issues/4621) but was unable to add the Gradebook tag.