The event generated when a node joins the group. A new instance of this
event is generated each time a node joins the group.
The event is generated on a "best effort" basis. It may not be generated,
for example, if the joining node was unable to communicate with the monitor
due to a network problem. The application must be resilient in the face of
such missing events.
The time when this node joins the group.
Returns the time at which the node joined the group.