You can assign students | customers to one or more Customer Groups via the Reg Form or via Student Details.
VIA REG FORM
First, you have to configure one of your Reg Forms to include the Customer Group question as an option. It is found under the Personal Information section of the Reg Form. Be sure to save the form after adding that field.
** Please note: You can control which Customer Groups appear in the dropdown on the reg form (for both students and office staff) by selecting or not selecting "Show Online" for each Customer Group via Configure | Customer Group. Go to Configure | Customer Group, select a Customer Group from the dropdown and see "Show Online". If this is checked to show, the group will appear in the dropdown on the reg form. If not, the only way to add it to a student record is via Student Details (below) or Configure | Customer Group. This is a great way to only display the relevant groups to students and office and hide the seldom used ones.
When you edit a student record or create a new student, be sure you select the appropriate Reg Form (e.g. maybe not the default one) so you can enter the Customer Groups.
VIA STUDENT DETAILS
Find the student and go to Student Details. Under Customer Groups, you can add or remove the student to one or more Customer Groups right from here.
ASSIGN MULTIPLE STUDENTS AT ONE TIME
You can assign multiple students to a Customer Group using Configure | Customer Group. You can filter for a course, class or individual students. Click the "+" to add the student(s) to the Customer Group and then click Save.