Paul's comment is correct. I just wanted to add some thoughts. "You being assigned to me" is the subject of the sentence, not just the word "you." If we look at the sentence even closer, we can see that "being assigned" is the true subject and that "You" and "to me" is information we can cut out: Being assigned is to set an example for people. I just cut out who is being assigned and to whom they are being assigned. Remember that the subject of the sentence is the thing acting in some way. If we rewrite the sentence like this: You is to set an example for people, we can see "you" isn't the one doing anything. In fact, the actual doer of the sentence is missing. You are being assigned by who? We don't know. The sentence doesn't say (passive voice, but I won't cover that now). This sentence is also an example of a gerund (-ing verb, here "being") acting like the subject. Because we can't typically separate the whole verb phrase, here "being assigned" stays the whole subject instead of just "being." Hope that helps!

Hmm it's very confusing. So the main subject is "the act", even though it is not stated in the sentence. Thank you so much. :)

"you being assigned to me" - take this as being one third person singular action: in other words "the act of you being assigned to me". therefore "is" is correct. https://www.italki.com/paul_curley