aCalendar is not involved in sharing or synchronization of your data, it's all done by your chosen cloud service.
To share calendars with friends, family or colleagues any good cloud service (e.g. google) can be used.
For write access, all users typically need to use the same cloud service, read access should also work across clouds.
For instructions on how to set up sharing, see the manual of your cloud service, e.g.:
- Sharing in Google Calendar (for Google calendars, you can also configure sharing directly in the Calendar List (manage calendars) in aCalendar)
- Sharing in Outlook.com