To join a talk as a guest, at the right hand side of the talk screen: either tap 'Join' or tap the microphone icon inside the circle. This will add you to the guest queue. Once you are in the queue, your guest request will be seen as pending under the guest circle, until the talker chooses to accept your request. When the talker accepts your request, a 3-second timer will count down to introduce you as a guest.
Please note: the talk host can decide to accept or reject your guest request. You may have to wait for any current guests to finish speaking, or for when the talker to decides accept you. If you navigate away from the talk screen, then you will no longer be in the queue.