SHOW GRANTS FOR `user`@`host`

create temporary table users (username varchar(128));

insert into users (username)
  select '\'user1\'@\'myhost\'' union all
  select '\'user2\'@\'myhost\'' union all
-- ...
  select '\'userN\'@\'myhost\'' ;

declare cursor mycursor
  as
  select username from users;

open mycursor;

fetch mycursor into @username;

while @username <> ''
  @sql = concat('grant {rights} to ',@username);
  prepare stmt from @sql;
  execute stmt;
  deallocate stmt;
  fetch mycursor into @username;
end while;

close mycursor;