You can manage and add new appointment types under Settings ➡ Calendar ➡ Appointment Types. Once you have created an appointment (how to create an appointment), you can make it a group appointment.
Creating a group appointment:
Go to Settings ➡ Calendar ➡ Appointment Types and make sure to check Group Appointment
Adding clients to a group appointment:
After you have added the appointment to your calendar, you can add clients as attendees.