For a more in-depth look into how to contribute to Xfce, as a developer, it is recommended to read The Ultimate Contributor's Guide to Xfce for more information.
There are various ways to get in touch with Xfce developers:
All Xfce developers have a real life and live across multiple time zones. If you don't get an answer straight away, be patient and stay online!
(The following list is extracted from The Ultimate Contributor's Guide to Xfce)