Depending on how they're joined to the call, your meeting attendees who aren't registered for Biba can appear a couple different ways.
Dialed in with organizer's PIN or a One Time PIN:
If they're dialed in using the organizer's PIN, the phone number they're calling from will be displayed.
If the dialed in user views a shared screen on the web viewer, they will also show on the roster as Viewer 1. Additional dialed in viewers will show as Viewer 2, Viewer 3, etc.
Dialed in on a Unique PIN:
You can choose to send out a unique PIN for each user in every meeting through your settings or preferences.
Because the personalized PINs are individually associated with a meeting attendee, Biba can show the email address of the dialed in user (if they're not registered), or their user name if they are registered.
They also use the same personalized PIN for their screen share url, so if an attendee is viewing a shared screen, it won't show as a separate line.