The Cloud SIG uses the #centos-devel IRC channel on Libera.Chat for ad-hoc communication and work coordination.; this channel is also bridged on Matrix as #centos-devel:fedoraproject.org. Official SIG meetings are also held on IRC.
See the CentOS IRC documentation for information on how to join IRC. For folks using Matrix we recommend following this guide.
For async discussions and announcements we use the centos-devel mailing list.