291186
Not easily...
The only thing you could do is modify your current theme file, so it displays your invitation somewhere, then remove your message after a while.
Displaying a popup to each user by this way would also be possible, but more difficult (you'd have to use a cookie so each user sees the popup only once).
Also, more and more people use anti pop-up software, so IMHO the "display within page" method is better.
Skalpa.>