Click on Resources.

Click on Add Room.

Choose a name.

Count - This is how many rooms will be created.  If I put 5 here, it will create Overflow room 1, Overflow room 2, Overflow room 3, Overflow room 4, Overflow room 5.

Enter the type of appointment.

Capacity - This is how many pets of the same family can be added to the room when adding the appointment.

Max weight - If you don't have a restriction, put a large number such as 350.  Otherwise if this needs to be size restrictive, put in the appropriate weight.

Private space - If this is checked, then only pets of the same family can be added to this room.  You will not be able to move pets from another family into these rooms in the Run Transfer screen.

Click Add when done.

Now you need to add the services that are available for these rooms.

Click on Service Configuration.

Click and drag over all services that apply to these rooms.

Now you are able to add appointments into these overflow rooms.