See ErcExtraModules first if you use the version of ERC that comes with Emacs 22 and you don’t have erc-list.el.
erc-list.el implements a channel list view mode buffer.
Use the /LIST command or invoke it interactively with M-x erc-chanlist RET to get a nicely sorted channel list in a separate buffer.
If point is at a channel name (at the # sign), you can join that channel by pressing
‘C-c C-j’. Use
‘s’ to sort alphabetically or according to the number of users in a channel. Type
‘t’ to toggle display of details about each channel. Type
‘q’ to hide the window and restore previous window configuration again.