You do not need any downtime to add a new node into P2P.
I hope you are planning to work out your implementation plan using a non-production system, so that when you make mistakes you can clear things down without affecting your business workload.
Original author: SQL Server FineBuild 1-click install and best practice configuration of SQL Server 2017 2016, 2014, 2012, 2008 R2, 2008 and 2005. 1 Dec 2016
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Disclaimer: All information provided is a personal opinion that may not match reality.
Quote: "When I give food to the poor they call me a saint. When I ask why they are poor they call me a communist." - Archbishop Hélder Câmara