I've noticed that anytime there is some sort of get together or event that requires members responding (RSVP) that it can often be somewhat of a hassle knowing just who all is coming.
On top of this, some embers may not see the thread/post announcing the event.
So here is an idea/option that might help.
Use the Site Calendar. In the lower right portion of the calendar is the option to "Create and Event".
Creating an event is as simple as posting a thread but you have a couple additional options: Date/Time of event and RSVP
If you choose to use the RSVP option, it will allow members that want to attend to select their RSVP status and this can be changed at any time to reflect changes in plans. A running count of who has RSVP'd will be listed with the event.
The Current/Upcoming events are also posted in the side columns of the forum for better visibility.
Here are two links that you can see what I am referring to.
http://www.aznlover.com/vbulletin/ca...y=2008-7-5&c=1
http://www.aznlover.com/vbulletin/ca...=2008-7-12&c=1
Hope this helps!