Although you can set up a course to include multiple sections and you can even constrain teachers and TAs such that they can only interact with students within their section. There is no way currently to prevent a teacher from adding a student into someone else's section. The +People form should constrain teachers and TAs such that they can only add new students into their own section (the section they were invited to teach).
Added to Theme
Reduce friction for large courses with multiple instructors Theme Status: Identified