There are several ways to contact developers and users of cairo.
If you've found a bug in cairo, you can discuss it on the firstname.lastname@example.org mailing list, (you will need to subscribe before the list will accept your mail). Or, if you prefer a web-based interface that lets you check back on the status of the bug, then enter a bug report against the cairo product at http://bugs.freedesktop.org.
Some people IRC. If you're one of them, you can find other like-minded cairo folk here:
server: irc.freenode.net channel: #cairo
If you think IRC is nothing but a big waste of time, feel free to ignore IRC since any significant discussion there will be reflected on the mailing lists. You'll just miss out on hearing things like how good Keith's latest pie was.