Mastodon is federated, meaning you can still follow users of BitcoinHackers.org but from a different Mastodon host, and still see their posts. What you don't get when you are not a BitcoinHackers.org user is the ability to see "Local" posts (which is kind of like a group that includes all users on the BitcoinHackers.org, which shows all messages from any bitcoinhackers.org user, even those who you don't follow).

I know at one time BitcoinHackers.org was seeing a large number of new accounts signing up at a rate that was too quick (and having large instances is not optimal, under the federated model) so the owner of BitcoinHackers.org (NVK) temporarily paused granting new invites. I suspect that pause is back on ... but this time it is probably not because growth of signups was growing too quickly.

In my opinion, Mastodon is a failed approach. It didn't solve censorship (many instances refuse to federate with BitcoinHackers.org), and for other reasons Mastodon is just not that very good. I still use it as I try to stay off of Twitter as much as possible, but my hope is that nostr or something else gives us a decentralized alternative to Twitter that doesn't suck.

We'll see.

In the meantime, you might try another Mastodon instance. I think Mstdn.social would probably be a good choice to join. If you later do eventually get an account on BitcoinHackers.org, you can "move" (redirect) your existing Mastodon account to your BitcoinHackers.org account, as described here:

Once you have a Mastodon account, you can start to follow users on any instance, including BitcoinHackers.org users. To help speed that part, here are instructions on how to obtain and import my list of Bitcoin-related users to follow:

Mastodon-Who-To-Follow https://cointastical.github.io/Mastodon-Who-To-Follow

It took me a few months to get approved too. It requires a mod (@nvk?) to manually let you in I think.