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.