ssh-keygen as another user

To set up an ssh-key usually, type
ssh-keygen -t rsa, hit [Enter] a few times, type
cp ~/.ssh/id_rsa.pub ~/.ssh/authorized_keys, and add entries from other places where you might have created a key, to the same authorized_keys.

To set it up so you can log in as another user (called, say, user2)
ssh-keygen -t rsa
Enter a different file to save (id_for_user2, say), and blank passphrase.
Now add the contents of the file id_for_user2.pub to user2‘s ~/.ssh/authorized_keys file.

Oh, and when you want to login as that user, you will have to specify that you want to use that file, like
ssh -i ~/.ssh/id_for_user2 user2@hostname. For long commands like this, it’s best to make an alias (put alias sshasuser2='ssh -i ~/.ssh/id_for_user2 user2@localhost' in your ~/.bashrc if all you want is a password-less replacement for su - user2). You can also give the id_for_user2 file to others, and they can also login as user2 without a password simply by typing the command above.

Got it from this linux.debian.user newsgroup post.