Hope you are doing well.
1. Generally this case do not happens but if you are facing such issue, you can again give permission to that user by adding him to the supergroup or you can create new user and give permissions.
2.Please follow below steps
sudo groupadd hadoop
Then create users you want. e.g useradd hduser
Then add this user to hadoop
Then edit hdfs-site.xml property and add this property dfs.permissions.superusergroup, value will be your group name
Now you can create any number of folders in hdfs and then assign ownership as yourusername:supergroup e.g hduser:hadoop by using below command
chown -R hduser:hadoop filename
Hope it resolves your query.
If you have any further query,please let us know.