I hold a "senior researcher" position at research dep. of one of security-related IT companies. I am developing a TLA model to specify one of companies product core protocols. My previous job title was "senior distributed systems developer"; I was doing a similar thing there.

I've yet to come across a TLA+-specific job title, although would love to be surprised. Usually it's the same people writing the spec, or engineers who produce a TLA+ spec from a provided informal-language spec.