Hi Matt,
We used to get fairly low turn outs of only a few people.
But in the last few years with the big resurgence in interest and nostalgia for retro machines, we are getting up to 20 people fairly often.
Check out the meeting reports at
http://saug.org.au for some photos of our meetings.
I think it's a great idea that you're kickstarting (no pun intended) a Perth user group, and I hope you can get some decent attendance!
In my experience it helps to promote the meetings on Facebook groups (like the Australian Vintage Computer Group), forums and anywhere else. I know of a couple WA based Amiga users.