According to the Physics Department website:
"LDS church members are expected to attend their local LDS church services. REU participants who are not LDS members are encouraged to participate in their own church services if they have one, but attendance is not required."

https://reu.byu.edu/faq#:~:text=LDS%20church%20members%20are%20expected,but%20attendance%20is%20not%20required.