It appears that the add event link is visible to anybody viewing the Calendar, even non-members/ non subscribers.
However I can not seem to allow certain users roles to add events.
These roles in UAM have Editor and Calendar_contributor checked and both read and write are set to all.
Logging in at this access level does not have the calendar option in the dashboard and clicking on the Add event link gives insufficient privileges error.
While the plugin functions very well, it is not useful if only an administrator can add events.