I am creating a custom registration script in php (registers in IPB and pM), and was wondering what the member_id field contains and the type of encryption its using. I would also like to know if there are any other fields/rows I need to worry about other than the basic username/pass/email fields.

The member_id is just a randomly created value that is used for unique identifying users.  There is not actual value encrypted to create it.

The screen name and the status are two other key fields that should be filled out for pMachine.