A conference call can occur between a maximum of four participants. To start a conference call, you need to be in a call with someone. Once you are in a call, tap the "New call" button, and select the next person you want to join. This will put the first participant on hold and dial the second participant. Once the call with the second participant is established, tap the Merge button to join both calls into one conference call.
By pressing the arrow in the top right corner of the screen, you can view a list of the participants, and enter private calls with them or remove participants from the call.