C is interesting as a language in being almost completely dependent on the system in which it is embedded. Many other languages are expected to provide facilities to the programmers which c leaves to the operating system. The result is that c is a very simple language, and very little discussion is needed regarding the particulars of the language. The communities using c that form tend instead to focus on the systems in use. You will find separate communities using c for kernel programming, embedded systems, and for higher level programs. If you are looking for a community then you should identify what you would like to do with c, and find a community specific to that.

That is an excellent and informative point. Bioinformatics is the subgroup that I am interested in for C, but that already tends to be pretty niche. The next best thing are the 'tool builders', of other command line tools. I've been enjoying reading the BSD utilities source code.

If I understand you correctly, there is one in UK - Association of C and C++ programmers association (https://www.accu.org/)

Most C developers I have met are not very active online. Talking about their work in person is also a mixed bag. Maybe they just don't like talking about it.