If have several Yahoo IDs, and use different IDs for different sets of people then you cannot chat with several at a time since at any point of time you would only be able to log into yahoo Messenger with only one Yahoo ID.

here is a software called M Multi Yahoo Messenger, that lets you log into Yahoo Messenger with more that on Yahoo ID, at the same time.

M Multi Yahoo Messenger works with any version of Yahoo Messenger, including the latest version and it does so by applying a patch to the Yahoo IM client software.

M Multi Yahoo Messenger Download

Windows : 98/Me/2000/XP/Vista