After registering, your account is not automatically activated. This requires human authorisation, which can take a few weeks. I only got approved after 7 days, but people have waited for months. While waiting, there's not much you can do - you certainly can't post, you can't send private messages, you can't even view others' profiles. Basically the same as a guest, and you can't contact anyone about it.
Oh, and links to most domains won't work, so if you get a message about domain not on the approved list, that's it.