how to get user permissions for nfs share?

Let usera be the primary group of UserA (groups usera). You can find out the GID using: id -g usera on the client system, say it is 1256. Now add your user to the usera group:

sudo adduser ayven usera

Now, on the server, do:

chown :1256 /Directory # Replace 1256 with the correct GID
chmod g+rwxs /Directory

Remount it on the client:

sudo umount /Directory
sudo mount /Directory

(You might have to relogin for your new groups to take effect.)

Now both you and usera have complete access to the NFS share. I have used the setgid bit, so you may have to enable it, I don't remember it. Add any users you wish to have access to this directory to the usera group.