Thank you for all the positive responses It's very encouraging :-)
Miaghstir: It seems to lock up when trying to send a message to someone who hasn't signed up yet (I was kinda hoping I could send a message so it'd be waiting for them when/if they do sign up).
Either that, or it's just really slow overall.
It's just being slow. I got a notice from my hosting provider saying that they are experiencing extremely high load at the moment. (not from us - it's a shared environment and the load is coming from someone using mysql). Hopefully it will clear up soon.
You can actually send messages to people who haven't signed up up yet. It shows you a warning before you try to write the message, but the message stills goes through and the recipient will get the message when he eventually does sign up.
So you can send a message to xxxxxx, see that he hasn't registered his account yet but send the message anyway, and then post a message on GOG telling xxxxxx that he has a PM waiting for him on GogPM and that he really should join.