Usage: USERHOST <nick> [<nick>]
Returns the user@host information of all nicks specified, you can pass up to five nick names as parameters.
/userhost Al Shadoe Gaz Bek
If you just want the address for some users this is a faster way to retrieve them than individually WHOISing each of them. It also only returns the mask, unlike the whois command that will several lines of information. You might want to userhost some people if you needed their mask to identify them.