I was about to accept your challenge, but the I realised that you were running invision.
it's tried and tested, not entierly hack proof, but a lot more work than I'm willing to put in!!
your only weaknesses will be your users really. I'd people using weak passwords or giving away passwords.
aside from that, there are a couple of cross site scripting vulenrabilities in the software you are using...
I didn't bother to sign up with an account, but will ask, can you have pictures in signatures? that's usually a place where exploits can be worked in.