When the call comes in, click the 3 dots and click transfer. Then chose the person you'd like to transfer to
from the list.
If you choose the option for warm transfer you can have a quick chat with the person you are transferring to so that they know who is on the line. To transfer to call to them, click at the top of the app where it says "2 active calls".
Now click the transfer button next to the original caller's name to transfer the call.