See ErcExtraModules first if you use the version of ERC that comes with Emacs 22 and you don’t have erc-nicklist.el.

To display channel nicknames in a side buffer, just add the following to your .emacs or ERC configuration file:

    (require 'erc-nicklist)

Then activate the nicklist buffer, using M-x erc-nicklist RET in the channel buffer (you need to do this for each channel buffer if you want a nicklist for each channel).

M-x erc-nicklist-quit closes the nicklist buffer.

ERC, ErcExtraModules