Sometimes it can be a pain to get the audio settings set just right. When you are in the room, can you see the audio meters moving at the bottom of the user cards for anyone?
If so, then Zencastr is getting access to their mic, but you just aren't hearing it. If not, then there is a mic configuration problem on their end.
Try having everyone ensure that the preferred input and output device is selected in the operating system preferences. Then just use the 'Default' option in Zencastr. You may need to completely close and reopen the Zencastr tab for the operating system setting changes to take effect.
This should ensure that the proper input and output devices are being used by Zencastr.
If no one can hear anything, then make sure that Zencastr's built-in VoIP isn't disabled in the recording settings. If this is off, then you won't be connected in a live VoIP call with any of your guests. Zencastr will simply record. This setting assumes that you are using a third-party VoIP provider like Skype or Hangouts.