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It's not "invite only". It's just a trick to seem exclusive. They have an automated email go out a few minutes after you request an invite that gives you instructions on how to sign up that seems like it's coming from an actual person (unless I'm mistaken, in which case whoever is managing these invites is superhuman).

Yup: http://blog.benhall.me.uk/2015/01/making-users-feel-special-...

If you wait for the animation to load, you'll see that it's free (even for enterprise features). They are owned by twitter (in case you're wondering how it could be free).

Crashlytics is free :-)