This chapter is from the book
Join a Server to the SharePoint Farm
Solution: Use the Join-SharePointFarm cmdlet from the SPModule.
The previous cmdlet (New-SharePointFarm) created the database and configured the SharePoint farm. You cannot run this again on the other servers. Instead, you need to join the servers to the farm using the Join-SharePointFarm cmdlet:
Join-SharePointFarm –databaseserver <database server name> -configurationdatabasename <config database> -passphrase (ConvertTo-SecureString -asplaintext -force -string "<farm pass phrase=")
The farm passphrase used with the Join-SharePointFarm cmdlet should be the same passphrase used when you are creating the new SharePoint farm.