Hi Speckle. I would not worry too much about it. My guess is that official registration is used mostly to have an idea of the amount of participants they can expect and make appropriate arrangements like food, venues, name tags, etc.
I would email someone from the school to let them know you will be around. I would find it very weird if they don't let you participate, you are an admitted student after all!
Did anyone else get a "deferred conditional admission"? They say it's "a strong presumption of admission to the class entering in 2019, conditional upon strengthening your quantitative skills."