wpmem_create_membership()

/**
 * This example would create a membership with the 
 * title "My Custom Membership", which, if unique
 * in the db, would result in a meta key of 
 * "my-custom-membership" and would have all other
 * properties set to the default values.
 */
$args = array( 'title' => 'My Custom Membership' );
wpmem_create_membership( $args );