There are several ways to contact developers and users of cairo.
Mailing Lists
This is the primary form of cairo-related communication. Feel free to come participate, or just lurk, on any of the cairo mailing lists. Archives: cairo, announce, commits, bugs.
Bug reports
If you've found a bug in cairo, you can discuss it on the cairo@cairographics.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 at https://gitlab.freedesktop.org/cairo/cairo.
IRC
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.