When I clicked the signup button to see what they would ask for, it took me to itch.io. I don't have an itch.io account, is that the only option to join?
Can I join the Go Godot Game Jam without an itch.io account?
Any problem with just creating an account? Itch is a good site.
Yes itch is a common/popular platform for game jams. And not just godot related ones. It's free to register an account on and for smaller games and projects, even plugins and/or resources you can also use it as a launch platform/store front.
As the others have mentioned, Itch.IO is free to sign up for and has many perks. I personally use Itch.IO as my primary way to upload games and projects.
That said though, it may be possible to participate without an Itch.IO account. To know for sure, you should ask those running the Go Godot Game Jam. I think any jam hosted on Itch.IO needs an Itch.IO account, but if they are hosting it internally or using another platform as well, then you may be able to sign up and participate without an account.