Web
Slate has a wiki..
Slate has a mailing list. The list is open to all to join or simply browse.
The mailing list is also visible (and postable) externally via GMANE.org gateway at http://blog.gmane.org/gmane.comp.lang.slate.general.
Chat
Slate users enjoy free discussion (almost) daily on the Freenode (formerly known as Open Projects Net) IRC network on channel #slate. Logs of #slate are available in "pretty" form and "raw" form.