There should be a "conf" softkey you can assign to it - I don't recall if it has join capabilities.
There's also a bug or more than one out there for this not working reliably anyways with this model of phone just for your reference, it will sometimes display "Invalid Conference Participant" or "Invalid Participant Info" and not work. This phone may be on its way out given the lack of firmware update, bug resolution, and the 8832 model on the horizon 