According to the Lobbyists' Code of Conduct, they are required to tell you who they represent, on whose behalf they are meeting with you. If they are consultants, they are required to tell you who their client is.
If you're going to meet with members of an association, I recommend that you make sure that your team provides you with the necessary information about them, whether they are in the registry.
I also recommend that members of associations make sure they are registered if they are not.