This error is due to the fact that before create the object doesn't exist. So, you can't pass it as parameter to the before commit event. You should use an after create event if you want to change members of the just created object.
I'm not entirely shure what you mean, but in you will probably be best of making a microflow that creates the object 'Member'. You can then manipulate that object, set references etc, and when you're done with these things, either commit the changes or open a form to show the newly created object (and let the user fill in the rest for instance).
@ Robert: No I mean that you replace the standard 'New' button with an Invoke button. You are then free to do perform any kind of action you wish, you can create the object, set info and then open a form (you are basically reinacting the default behaviour of the Mendix Business Modeler with microflows, which I personally prefer because you have a lot more freedom then)