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 and click lick 'Manage students'.
Search for students by name, or use the filters to find the correct year/grade, group, or gender if you are bulk adding students.
Click 'Search' to display the students that match the filter criteria, then click the green '+' icon to add them. Finally, click 'Save'."
This will add the students to the group and also all of the attendance registers.