Step 1: Log in to the server as a sudo user, such as ubuntu

Suppose you are user abc.

Suppose you have this segment in ~/.ssh/config on your local machine:

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Host datanode1
    user ubuntu
    hostname 123.compute-1.amazonaws.com
    IdentityFile ~/.ssh/id_rsa

and you have been granted access to datanode1 as sudo user ubuntu, which means your public key—the content of your ~/.ssh/id_rsa.pub is pasted in /home/ubuntu/.ssh/authorized_keys on the server node datanode1.

Now log into datanode1:

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abc @ local $ ssh datanode1
Last login: Mon Apr  1 01:01:01 2017 from abc.def.com
ubuntu@ip-1-2-3-4:~ $ 

Step 2: Create user on server

Now as sudo user ubuntu logged in on datanode1, create a user abc:

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ubuntu@ip-1-2-3-4:~ $ sudo adduser abc

This command will ask for some info. Set a password (such as 1234) and leave the other things blank.

Add the new user to the sudo group:

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ubuntu@ip-1-2-3-4:~ $ sudo usermod -aG sudo abc

Add the new user to the docker group (if docker has been installed on this server):

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ubuntu@ip-1-2-3-4:~ $ sudo usermod -aG docker abc

Step 3: Grant connection access

Still as ubuntu, do

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ubuntu@ip-1-2-3-4:~ $ cd /home/abc
ubuntu@ip-1-2-3-4:/home/abc $ sudo mkdir .ssh
ubuntu@ip-1-2-3-4:/home/abc $ cd .ssh
ubuntu@ip-1-2-3-4:/home/abc/.ssh $ sudo vim authorized_keys

Now paste the content of ~/.ssh/id_rsa.pub on your local machine into this new file, and save it.

Now /home/abc/.ssh and /home/abc/.ssh/authorized_keys are owned by root. Hand them to abc:

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ubuntu@ip-1-2-3-4:/home/abc/.ssh $ sudo chown abc:abc authorized_keys
ubuntu@ip-1-2-3-4:/home/abc/.ssh $ cd ..
ubuntu@ip-1-2-3-4:/home/abc $ sudo chown abc:abc .ssh

Step 4: Log in to the server as user abc

Now on your local machine, change the ~/.ssh/config segment to

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Host datanode1
    user abc
    hostname 123.compute-1.amazonaws.com
    IdentityFile ~/.ssh/id_rsa

Then you can log into the server as user abc:

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abc @ local $ ssh dn1
Last login: Mon Apr  1 02:01:1 2017 from abc.def.com

abc@ip-1-2-3-4: ~ $

Next you do is set up and customize your account on the server

Ideally, steps 1, 2, 3 are done for you by a sys admin, and you pick up at step 4.