Call your first person. When they answer, tap "Add Call"
Choose your second person.
It will automatically put Person 1 on hold, and call Person 2. It will not let you "Merge Calls" until Person 2 has answered.
Now that Person 2 has answered and Person 1 is on hold, you just tap "Merge Calls"
If you'd like, you can add more people by repeating the steps.
Thanks! I hoped this helped.