Communication for the Zope Toolkit development itself happens both on IRC and in a mailing list.
We use #zope on the IRC network provided by freenode.net.
The Zope developer community gathers every Tuesday at 15:00 to 15:30 UTC on #zope to discuss current matters.
Agendas and summaries of the meetings are sent to the zope-dev mailing list.
The official Zope Toolkit development mailing list is zope-dev@zope.org. You can subscribe to it by visiting http://mail.zope.org/mailman/listinfo/zope-dev.
Note
In the earlier days of Zope 3 development we used to have a separate mailing list for development on Zope 3 (zope3-dev@zope.org). This list was merged into the general zope-dev@zope.org list a while ago.