You can set availability for your Team Members in Settings.
Or go to Settings > Team Members > Select Team Member > Availability
By default, if Availability has not been set for a Team Member, all times on their calendar can be booked and there will be no greyed out blocks.
Click 'Add More Hours'
Select the times that Team Member is available, or Edit existing hours set
Once set when a team member is unavailable they will be greyed out of the calendar.
When on the calendar, if an appointment is attempted to be added in any unavailable time a pop up warning will be displayed, confirming that they are unavailable. This warning can be closed, or you can create the appointment anyway. This gives the best control when creating appointments when team members are unavailable.