Adding new node will there be any performance issue or downtime?

Have to add a new node without any downtime. How to achieve ?

Set it up like the other ones and let it join the cluster, there is no downtime.

You can get some performance degradation caused by shard relocation to new node (cluster will rebalance if new node appears). If it's valueable for you, you should limit numbers of concurrent shard relocations or temporary disable it and make a manual shard relocation.