From my experience, most physics research groups should be more than happy to let you use what tools you want, and I suspect most good ones would appreciate the benefits of Julia. Granted, depending on what you’re doing there may be large existing code bases (I worked on CMS which had a pretty sizable C++ code base, much of which I think also had Python wrappers by the time I was there, and of course use of ROOT in the HEP community is ubiquitous). I’d still expect that most groups would be happy to let you use Julia as long as you can make use of existing API’s, so Cxx.jl could be an important factor. Of course, there will inevitably be groups in which your primary job as a grad student will be to work on existing code (I’d imagine most groups which are primarily focused on computational physics are like this), in which case you’ll have fewer options, but you should have a good idea before joining a group whether you’d be stuck in that kind of situation.
Anyway, it would of course be great if people chime in with information about specific groups using Julia, but I’m posting here to recommend that you not worry about this aspect of choosing a group too much. Physicists tend to appreciate nice things, and in most cases your advisor is not likely to be overly opinionated about how you are writing your code, as long as you get your work done. It’s if/when you have the misfortune to go into private industry that you’ll be well and truly screwed (but less poor). In the ivory tower, enjoy using Julia on whatever.
(Of course, this is just my personal experience specifically in high energy physics. I’d imagine things are similar in astro/cosmo, I really have no clue what it’s like in the condensed matter or AMO worlds.)
By the way, updating ROOT.jl would be a really good start to getting to work with Julia during your career in HEP or accelerator physics