// this should be a duplicate of bp_groupblog_signup_form(),
// but with your mods to add in your custom roles
function my_bp_groupblog_signup_blog() {
}

function my_override_signup_blog_form() {
    // remove current signup blog from
    remove_action( current_filter(), 'bp_groupblog_signup_blog' );

    // add your custom signup blog form
    add_action( current_filter(), 'my_bp_groupblog_signup_blog' );
}
add_action( 'bp_groupblog_create_screen_markup', 'my_override_signup_blog_form', 20 );
add_action( 'bp_groupblog_edit_screen_markup',   'my_override_signup_blog_form', 20 );