With GSM, you can set up a conference call with up to five people (depending on your carrier).
Create a conference call. While on a call, tap Add Call, make another call, then tap Merge Calls. Repeat to add more people to the conference.
Drop one person: Tap next to a person, then tap End.
Talk privately with one person: Tap , then tap Private next to the person. Tap Merge Calls to resume the conference.
Add an incoming caller: Tap Hold Call + Answer, then tap Merge Calls.