I found out about Lavaboom at
https://www.privacytools.io/, which in itself is worth a look.
do you have an invite code?
No, but it looks like anyone can reserve a username at this point if you provide them with another email address. I got this:
Nice one!
Your username has been reserved.
We have just sent an email to
******@protonmail.ch
We are currently in a closed beta and rolling out account invitations gradually. The email you will receive will tell you how long the wait time is.
I've not received an email yet though. I think when I get the email it may have an invite code in it because the next screen after the one I quoted above allows you to 'Verify invite code'.