Using them on node types

Each nested class is a property type. Name the property whatever reads best:

public interface IVenue {
    // ...
    EventsAtVenue.Events Events { get; }          // the venue's events
}

public interface IEvent {
    // ...
    EventsAtVenue.Venue  Venue     { get; }       // the event's venue
    Attendance.Attendees Attendees { get; }
}

public interface IAttendee {
    // ...
    Attendance.Events Attending { get; }
    Friends.Peers     Friends   { get; }
}

Declare only the ends you navigate. You never assign an end: relate nodes with AddRelation, SetRelation and the other verbs in §14. On a class, initialise each end with new().