There is a simple way how to make your Yahoo Messenger can be used for multiple user.
If you are using Microsoft Windows XP, you may try this steps:
1. Go to regedit editor.
From the keyboard: hold windows button then pres R (run) then type regedit and OK.
2. Find this registry:
HKEY_CURRENT_USER --> Software --> Yahoo --> Pager --> Test
On the right side, right click your mouse and select option: New –> DWORD value
Rename the New Value # to become: Plural
Double click Plural and enter value data: 1