Unfortunately they're not allowed to do that, well not at this stage anyway.
My understanding is that once it actually gets filed in the courts, then all shareholders on the register would be notified, and at that point I would expect many more to sign up.
At the moment it's just Piper Alderman taking expressions of interest so that they can judge if there's enough shareholders out there to make it viable for them to proceed. They can't use the share register to canvas interest, which is a shame as would be much easier!
