Support collaborative scheduling for groups

Is your feature request related to a problem? Please describe.
Appointment currently works well for a single individual using another individual's availability to schedule a meeting, but not for multiple individuals to collaboratively schedule meetings.

Describe the solution you'd like
I'd like to either integrate my full calendar, or make a sample weekly calendar which shows my 24/7 availability. I'd be able to share this with a group of people who can also input their own calendar or sample weekly availability. Then the system would highlight what day + time would suit the most people.

Describe alternatives you've considered
Framadate, Rallly and When2meet but their UI either leaves a lot to be desired, they lack multi-timezone support or they don't allow people inputting what their 24/7 availability is.

Additional context
My usecase is for scheduling recurring meetings (weekly or fortnightly), across individuals in multiple timezones, and wanting to at a glance see what time slot fits the most people.

No files, tests, or entry points are named. Start by locating the existing individual availability and appointment-scheduling flows, then determine how group availability, recurring meetings, 24/7 weekly schedules, and multiple time zones should be represented; done means showing the time slots that suit the most participants.