how to setup AWS/mongodb with replicas so that if an instance crashes there is nothing to do?

how to setup AWS/mongodb with replicas so that if an instance crashes
there is nothing to do?

The question is pretty much all in the title : how to setup AWS/mongodb
with replicas so that if an instance crashes there is nothing to do ?
Namely, if one of the instances of the replica crashes, I guess that
Amazon instantiates a new machine for us and starts up the processes that
were running. With EBS things should be fine.
Only problem : when restarting this, how can we add back the new machine
to the replica set ? Instances have changing ips and Im not sure how how
start up the machine will know what replica set to join, what will be its
ip, and how to tell to join it.
If you have encountered this problem please let me know !
Thanks
Thomas