Place a call to the first participant.
When you're connected, tap Add call  and select
the contact you wish to add. You can also tap the Dialpad
 and select
the contact you wish to add. You can also tap the Dialpad  then add call.
 then add call.
After you’re connected, tap Merge call  . The participant is added to the conference call.
. The participant is added to the conference call.

Tap here to merge the separate calls into a conference call.