To share calendars with friends, family or colleagues any good cloud service 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 (you can also configure sharing of Google Calendars directly in the calendar list in aCalendar)
- Sharing in Outlook.com
aCalendar is not involved in sharing or synchronization of your data