I have created a basic set-up and some of us have log’d in fine, but others are not having much luck. It appears that when they click the Activation link in the email they get sent – it asks for a Valitation Key – but none is supplied on the email – and they are unable to join in.
I recently ran into this issue as well and found that troubleshooting the plugins associated with the site resolved the issue. There was a specific plugin that was causing the problem. I simply deactivated them and activated them by trial an error until I isolated the culprit. I realize this is an older post but maybe someone will benefit from this solution.