erc-netjoin-hook is a variable defined in `erc-netsplit.el'.
Its value is
nil
This variable may be risky if used as a file-local variable.
Documentation:
Run whenever a netjoin is detected the first time.
Args: PROC is the process the netjoin originated from and
SPLIT is the netsplit (e.g. "server.name.1 server.name.2").
You can customize this variable.