Email Discussion Group
The default means of communication within the OpenCog community is the "OpenCog mailing list" aka Google Group. You can find the archives, and/or sign up, here:
Discussion also frequently takes place within different repos on GitHub, in the form of comments on various GitHub issues or pull-requests. You can browse these on the OpenCog GitHub repository, or subscribe on GitHub to receive them via email.
There is also an IRC channel which has been widely used at various points in OpenCog's history. Chat on channel #opencog on Freenode IRC (web chat).