Attendance has hovered (for the prime-time US ones) around 40-50 players, but there is variance. For ones run at non-US-prime time (like Europe prime time or Australia prime time), they have lower attendance.
It's own forum won't do anything to attendance. Few people read the special forums except for people who are already attending.
I advertise these things in the places where people who don't know about them tend to be more prevalent: the general forum.
The thing that is most effective in recruiting is people recruiting their friends and squaddies to play and talking it up in the Main Arena when the opportunity arrises.
So, if you like them and want more people to play, spread the word. That is by far the most-effective route.