The following bug has been logged on the website:
Bug reference:      18137
Logged by:          TAKATSUKA Haruka
Email address:      harukat@sraoss.co.jp
PostgreSQL version: 16.0
Operating system:   any
Description:
In the following release note item, it seems that "GRANT group_name TO
user_name"
should be "ALTER GROUP group_name ADD USER user_name".
As far as I have tested and confirmed,  ALTER GROUP is the one that
has been allowed since version 16.
test steps:
db1=# CREATE ROLE usr1 LOGIN;
db1=# CREATE ROLE usr2 LOGIN;
db1=# CREATE ROLE usr3 LOGIN;
db1=# CREATE ROLE grp1;
db1=# GRANT grp1 TO usr1 WITH ADMIN OPTION;
db1=# \c db1 usr1
db1=> ALTER GROUP grp1 ADD USER usr2;
   15.x: ERROR:  permission denied
   16.0: GRANT  successfully
db1=> GRANT grp1 TO usr3;
   15.x: GRANT  successfully
   16.0: GRANT  successfully
release note item (release-16.sgml 963) :
<!--
Author: Robert Haas <rhaas@postgresql.org>
2022-08-22 [ce6b672e4] Make role grant system more consistent with other
privil
-->
      <listitem>
       <para>
        Allow <link linkend="sql-grant"><literal>GRANT group_name TO
        user_name</literal></link> to be performed with <literal>ADMIN
        OPTION</literal> (Robert Haas)
       </para>
       <para>
        Previously <literal>CREATEROLE</literal> permission was required.
       </para>
      </listitem>