What sort of professional boundary should I set with other students when I am a grader for a class I will be taking?
Actually, I think the best course of action would be for you to decline this offer. You have different personal relationships with different people in the class and it will be difficult to put aside all of your unconscious feelings and grade in a completely unbiased manner. It also opens the possibility of your friends putting pressure on you, which you can resist, but which could become uncomfortable.
An exception would be for grading that is purely objective, not requiring any analysis or judgement of the quality of the answers, such as mechanically recording the answers in multiple choice questions.
However, if blind grading could be arranged so that you grade paper not knowing who wrote them, then my reservations would probably disappear.
Blind grading would leave you free to interact normally with study groups and would lessen or remove any pressure that could be applied against you.
At a minimum bring up such concerns with the professor and point out the uncomfortable position it puts you in.
Perhaps there is a possibility that you help out the prof by grading a different class and suggesting you grade this one in a future offering.
Just to go against the grain, if you decide to take it:
Be methodical and very objective. Every action you take should be documented and based on provable facts.
I taught friends for more than a few courses. I usually do a pass cataloging errors, then I assign a mark for each error, which leads to the grades. That way, when someone questions the markings (Brazilian national sport at universities btw), you can literally point at the rules and how the grade was constructed. A friend that was letting me crash in his couch for the duration failed one of my courses and we are still friends...
For the day to day stuff, since you don't have access to anything confidential, just be a better than average student that helps people, whenever you feel like doing it, but at least during your TA hours. Be careful with the ethical lines (like if your professor mentions something that is not public knowledge), but don't overthink it.
I have some experience with this kind of situation. Two or three times in my career as a professor a student was in a situation similar to yours: too much knowledge to take the course but for various good reasons needed to. I arranged for them to (help me) prepare solutions to hand out and sometimes grade papers.
Everyone in the class and in the department knew about this arrangement. It never seemed to cause any problem. My general attitude toward homework may have helped ease any potential friction - the purpose of the assignments was to encourage learning, not to assess. So they were commented on extensively, marked just [excellent|done|please see me for help] and did not count heavily toward the final grade.
Your situation may vary.