To add a student to a classroom or group go to Administration, then to Groups. Next, select the classroom or group you'd like to add a student to.
Scroll down to the bottom of the Students section, then click the + icon to add a student. A menu will open on the right side of the screen, from which you can filter students by name or by group. Once you've found the student you'd like to add, click the blue + icon to the right of the student's name to add them to the classroom or group.
Make sure to click Save Group to save your changes once you are done!