Well, I tried to answer an ask via mobile, since I couldn't access the inbox on my desktop, but then tumblr crashed, so I don't think that's going to happen. Let's just address the issues so far here:
First of allâearlier I set it so that sign-ups would open an hour ago, so that time has passed, and for me the page is saying that sign-ups are closed. Can anyone check if there's a "Sign-up Form" button on the side that you can fill out? That would be really helpful to know whether or not that works from a non-admin account! Since I was able to sign up, if you see the form button, you should be able to as well!
For now, you can click the "Join" button, but that doesn't really do anything as far as sign-ups are concerned.
@oyasumi-artykun, thanks for joining! Hopefully you'll be able to sign up soon too. Here's a quick run-down of what an offer is: basically, AO3 has a system that automatically matches you based on your prompts and offers. Prompts are what you, as an author, will actually have to take into consideration. Your offers are things that you're willing to write, and that's what AO3's matching algorithm uses to give you prompts. You format them just like a prompt, but you probably won't end up writing exactly what you offer. I hope that makes things clearer!