Whenever you like you can choose to import additional students or teachers to Loops through the import functionality. You do the same thing you did the first time. The only difference is that you only add the new participants that you want to invite to Loops in your excel sheet before import. As before, you can also choose to divide the new students and teachers into groups already in the import. If you want to remind yourself about the procedure, read more about it in the article Import all students (and teachers) to Loops at once.
