This should've been put in the scripts & pop-ups section, as that's the only way to do it. There isn't a single command that does what you're looking for, but the following script does.
Some IRCd's do have a Greet option, which goes to the person's status window when they join a room. This option, if available, is run through chanserv and has to be configured by the founder. Since not all networks have chanserv, and not all chanserv's have the Greet option, I gave him a quick code that'll work under most conditions.