At your school, you may have applicants who will start in September join you for taster days in the preceding Summer term.
If you have added your students to your site already as applicants, you should not enrol these students into your school early. This would cause you to output incorrect student numbers overall, and for students in each year group in the census.
You won't be able to put them onto registers in Arbor and you'd need to take attendance for these children outside of Arbor. If you already have them set up as Applicants, you can download a list of them from Students > All Students > Applicants by selecting the intake season and going to Applicants from the left-hand menu.