Just your last initial is fine, or you can put N/A, if you do not wish to disclose. This field cannot be disabled in the software, nor made optional, so you have to put something there.
This is the date of the day following your last drink or use.
Your belly button (AKA when you were born) birthday, if you wish to include it.
Your email address is not visible to other users.
This determines your level of access to group documents and administration permissions.