Don't do it
You are the wrong side of legal -
If you want to claim the hours on a group-owned aircraft, where you are not a member, then pay up.
It doesn't matter that the group's paperwork is 'unofficial'. It will prove to be evidence in any further claim.
Just remember that if anything went wrong and the CAA check your logbook and that of your friend and the group's records -and they would - the insurance might be ruled invalid.