Add a user to a group via API?

Something similar to this will add users “bill” and “ben” to group “flower_pot_men”:

	/**
	 * Add users to a specified group, create the group if it doesn't exist.
	 *
	 * @param string $group_name name of group
	 * @param array  $usernames  users to add to group
	 *
	 * @return mixed HTTP return code and API return object
	 */

	public function add_users_to_group( $group_name, $usernames = array() ) {
		$group_id = null;

		$result = $this->get_request( "/admin/groups.json" );
		if ( $result->http_code != 200 ) {
			return $result;
		}

		foreach ( $result->result as $group ) {
			if ( $group->name === $group_name ) {
				$group_id = $group->id;
				break;
			}
			$group_id = false;
		}

		if ( ! $group_id ) {
			$result = $this->post_request( '/admin/groups', array( 'name' => $group_name ) );
			if ( $result->http_code != 200 ) {
				return $result;
			}
			$group_id = $result->result->basic_group->id;
		}
		if ( ! empty( $usernames ) ) {
			$params = array(
				'usernames' => implode( ',', $usernames )
			);
			$result = $this->put_request( '/admin/groups/' . $group_id . '/members.json', $params );
		}

		return $result;
	}
$group_name = 'flower_pot_men';
$usernames = array( 'bill', 'ben' );
$result = $api->add_users_to_group( $group_name, $usernames );
if ( $result->http_code != 200 ) {
	// @todo handle the error
}

EDIT: The key point here is the /admin part is different from yours.
EDIT #2: Updated to correctly reflect ID usage.

More about Bill and Ben

“Wubb-ooooh”