Community

#c on Libera.Chat
IRC channel on Libera.Chat; previously #c and ##c on Freenode. visit wiki for more details
ACCU
While nowadays its interests encompass a wide range of languages and topics, previously it was known as Association of C and C++ Users.
C Board
Forum accompanying Cprogramming.com website.
C/C++ Users Group
A (now historical) group focused on low-cost distribution of open-source C/C++ code. CUG also launched C/C++ Users Journal.
Some of their CD-ROMs can be found archived on Internet Archive.
Codidact
Software development Q&A platform with dedicated C tag.
Comprehensive C Archive Network
Beside the repository itself, there are mailing list and #ccan channel on Freenode. CCAN is inspired by Perl's CPAN
DEV Community
Social network for software developers with dedicated C tag.
Discord
This communicator has several servers dedicated to the C language, from publicly well known you can join:
ISO/IEC JTC1/SC22/WG14
The group of volunteers using their expertise to draft new revisions of the C standard.
Lobsters
A link aggregation and discussion board with tag dedicated to C programming. posting only after an invite-only registration
Meetup
This social networking platform for creating local groups and events has a dedicated C & C++ topic tag.
Reddit
This social networking platform hosts many programming communities, including few dedicated to the C language:
Stack Overflow
Popular software development Q&A website with dedicated C tag.
Usenet newsgroups
There are prominent newsgroups about C language:
Worldwide users' groups
After all, C is an international standard!