erc-join-buffer

erc-join-buffer is a variable defined in `erc.el'.
Its value is

buffer

Documentation:
Determines how to display a newly created IRC buffer.

The available choices are:

'window - in another window,
'window-noselect - in another window, but don't select that one,
'frame - in another frame,
'bury - bury it in a new buffer,
'buffer - in place of the current buffer,
any other value - in place of the current buffer.

You can customize this variable.