Is it possible to associate multiple key pairs to the EC2 instance that I can give to other people for access?
Yes you can. You should however get the other people to generate their own key-pairs and then have them give you their public key.
You can then add their public key to the
The main reason to do it this way is that it saves you having to work out how to securely transfer the private key you generate (and it's pass phrase) to the person you want to use it. There is no problem with transferring public keys, they are after all public.