I need to monitor number of the facebook group users and display it on the website. I know that it is possible to get User IDs using their API, but they are limited to 500 only (if the total number of members is 500+).
What would be the easiest way to get total number of members that signed up to a Facebook Group that I'd set up? Is this at all possible?
If you write an http bot, it shouldn't be very hard to scrap, given that real-time performance is not the key.
According to the documentation for Groups.getMembers it isn't possible to get > 500 group members with an API call. Worse, you seem to only be able to get 500 random members.
You may want to consider using Facebook Connect with your site instead. I'm no expert on Connect but I believe you wont have this problem using it since you are actually writing Facebook-specific code -- seems like there would be no purpose in limiting results. That'd be the direction I'd look, at least.
Good luck.