TimeTree offers two different sharing options.
One option is a shared "Calendar", the other is a shared "Event".
Note: How to create a shared calendar and a shared event are different.
Convenient for sharing multiple events with different groups such as family, friends, coworkers, etc..
To create a shared calendar, invite members on Calendar More tab. (No an account registration required)
Convenient for sharing one-time event such as a party. An event host can select one group at a time and can be changed per event.
To create a shared event, invite members from Friends tab (An account registration should be required.)
-Friends List for Shared Events
This allows you to create a list and add members whom you often share events with