One can add a room to auto-join list (by right clicking it in the room browser and adding as bookmark and then marking "Join on startup" option. But if a room is password protected, user will be greeted with password field upon launching Spark. Maybe Spark can store room password in spark.properties file in encrypted format like it does with login passwords. Maybe there can also be an option to remember password when joining a room manually.
Update: recently an option to save Room's password has been added to Spark. Renaming this ticket to make Spark automatically join the room with the saved password, eliminating the need to press the OK on the Password required dialog.