The more the merrier. To invite friends to a Chat room:

Click Chat in the Chat window and select Invite More People.

Double-click the friends you want to add, or highlight their names and click the Add button. Add as many friends as you like. (If you want to remove one, just highlight their name in the box on the right and click Remove.) You can invite someone who is not on your Messenger List by pressing the Invite Other button and entering their Yahoo! ID. (Your friend must have Messenger installed and running).

Enter your optional welcome message and click the Invite button.