Yes the keypair is tied to the ubuntu user. After you create the new users, you'll need to put the key you want to use in
$HOME/.ssh/authorized_keys .. Its a little complicated, as you also need to make sure the $HOME/.ssh has secure permissions
chmod 0700 $HOME/.ssh
and that the authorized_keys file also has secure permissions
chmod 0600 $HOME/.ssh/authorized_keys
One trick is to just copy the ubuntu user's key file to the new users if you just want to use the same key.
sudo cp -arf /home/ubuntu/.ssh /home/myuser/.ssh
chown -R myuser.myuser /home/myuser/.ssh
If you setup a password for those new users (not recommended actually.. password brute force attacks of SSH servers are rampant on the internet) you can also do this from your client machine:
Which will handle all of the permissions for you. Also if you store your key in your Launchpad.net account, you can do
And it will pull your key from launchpad (this command was renamed to ssh-import-id in 11.04 and later).