For your MAT to be able to aggregate data from all their schools on their Group MIS site, they will send you a data sharing request. To confirm the request, go to School > Data > Data Sharing.
You'll need to have the School: Action: Data Sharing Request: Administer permission to access the page and confirm the request. Only the Head Teacher has this permission by default.
Click into the request and click Accept to confirm and share your school data with your MAT's Group MIS site.