Find and allow ssh clients
 

Is there any way to find out from this machine, this user is logged into my machine using any mechanism or command?

Also I can allow this users can put ssh to my system. Is it possible?

Not sure what are you asking, but you can view the currently logged in users with w or who command.

Yes It is possible

Find and allow the ssh clients
 

I too find out using SSH_CLIENT and SSH_CONNECTIONS environment variable it could be done.

Is there any other way?

I like the following command:
netstat -a | grep ESTABLISHED
It's not for showing users or specifically ssh, but useful for exploring connections...

It certainly is. If you're running sshd (ssh daemon) anyone with a username and password can login unless you've locked them out. if you are on a router or switch, or hub, you need to port forward port 22 on that device. that is ssh's port usually. goto this address: http://www.whatismyip.com/
and have the user input that address. i'd use
If you're not running sshd type sshd in the terminal. If you don't have it installed, go into your package manager and get it. although every system i've had has shipped with it.