New secondary replica to AlwayOn

Already existing AlwaysOn 2012 running with Primary+1Secondary replica(Manual Failover)

Now we are planning to add one more secondary replica with Automatic failover.

This new node is already a part of the cluster and installed sql server, now the only pending task is to add to AlwayOn.

How to add this new node to alwaysOn and sync database from primary.

Does it requires any downtime because this is PRODUCTION?

Please suggest.