Crashlytics was acquired by Twitter, and now seems to have been rolled in to their Fabric integrated mobile development service.
None of the Crashlytics signup links seem to work any more. They all redirect me to the home page. Very confusing.
You should instead sign up for Fabric:
https://fabric.io/
When I was invited to a Crashlytics account, all I received was the cryptic email below.
I thought this was some weird spam from Twitter that I had forgotten about signing up for. There's no mention of it being related to Crashlytics or to the active invite that I looking for.
However, if you accept the Fabric invite you will receive access to Crashlytics.