There is no recovery function for those, the group username and password can only be setup and changed by the group administrator. To do so, log in as the group administrator and change them in the group administration page.
The group administrator’s and the group’s username/password are different accounts, and you cannot use one password for the other. These are often confused, leading to such scenarios where the group administrator requests a new password using his/her email address then tries to use the new password with the group username.