scp - How do you manage ssh keys to add a second user?

I used this article to set up keys and a user to login to a Ubuntu server from a Windows box using Putty.

I would now like to add an additional ssh user that will login from a MacBook. I used ssh-keygen to generate the local keys but I fail in getting the keys copied up to the server. Here is my bash readout (changed in parts for obvious reasons)

MacBook-Pro:~ joe$ scp -2 -P 50022 -v ~/.ssh/id_rsa.pub newuser@111.222.333.444:
Executing: program /usr/bin/ssh host 111.222.333.444, user newuser, command scp -v -t .
OpenSSH_5.2p1, OpenSSL 0.9.8l 5 Nov 2009
debug1: Reading configuration data /etc/ssh_config
debug1: Connecting to 111.222.333.444 [111.222.333.444] port 50022.
debug1: Connection established.
debug1: identity file /Users/joe/.ssh/id_rsa type 1
debug1: identity file /Users/joe/.ssh/id_dsa type -1
debug1: Remote protocol version 2.0, remote software version OpenSSH_4.3
debug1: match: OpenSSH_4.3 pat OpenSSH_4*
debug1: Enabling compatibility mode for protocol 2.0
debug1: Local version string SSH-2.0-OpenSSH_5.2
debug1: SSH2_MSG_KEXINIT sent
debug1: SSH2_MSG_KEXINIT received
debug1: kex: server->client aes128-ctr hmac-md5 none
debug1: kex: client->server aes128-ctr hmac-md5 none
debug1: SSH2_MSG_KEX_DH_GEX_REQUEST(1024<1024<8192) sent
debug1: expecting SSH2_MSG_KEX_DH_GEX_GROUP
debug1: SSH2_MSG_KEX_DH_GEX_INIT sent
debug1: expecting SSH2_MSG_KEX_DH_GEX_REPLY
debug1: Host '[111.222.333.444]:50000' is known and matches the RSA host key.
debug1: Found key in /Users/joe/.ssh/known_hosts:2
debug1: ssh_rsa_verify: signature correct
debug1: SSH2_MSG_NEWKEYS sent
debug1: expecting SSH2_MSG_NEWKEYS
debug1: SSH2_MSG_NEWKEYS received
debug1: SSH2_MSG_SERVICE_REQUEST sent
debug1: SSH2_MSG_SERVICE_ACCEPT received
debug1: Authentications that can continue: publickey
debug1: Next authentication method: publickey
debug1: Offering public key: /Users/joe/.ssh/id_rsa
debug1: Authentications that can continue: publickey
debug1: Trying private key: /Users/joe/.ssh/id_dsa
debug1: No more authentication methods to try.
Permission denied (publickey).

lost connection