irssi/docs/help/in/join.in

@SYNTAX:join@

   -window: Join channel, placing channel window item in currently active window
   -invite: Join last invited channel
   -<server tag>: Join channel on server with specified tag

Joins a specified channel. Channel names usually begin with #-sign,
which may be omitted here.

JOIN is aliased to J by default. Example: /j irssi
(This joins to the channel #irssi)

See also: PART, WINDOW CLOSE