You will need to have Admin permissions. Once you have allocated students and generated events you will then be able to 'Manage Students' to add to or remove students from an activity.
Go to the container and find the relevant activity:
Click 'Manage students'. Use the filters to find the right grades, genders etc and then click the green + sign to add the students. Then click 'Save'.
This will add the students to the group and also all of the attendance registers.