If you have a lot of buddies in a forum (let's say 5), then it is easier to bring out your Buddy List and tick each of the 5 persons and press PM users. Then a brand new message (ready to be written) will appear in another browser window or another tab in your browser. Let's take it step by step:
Left click
Quick Links in the aom3.org bar (see attachment).
Left click
Open Buddy List (see the same attachment)
Tick the 5 buddies in the Buddy List browser window that now opens and press
PM users (see attachment).
Now you can write the message once instead of writing the same one five times (see attachment)
Note: I know this is only one way to do this manoeuvre but it's as easy as anyone